Theming Shiny Apps with bslib

Customize Shiny app appearance using bslib's Bootstrap 5 theming system. From quick Bootswatch themes to advanced Sass customization and dynamic color mode switching.

Quick Start

"shiny" preset (recommended starting point):

page_sidebar(
  theme = bs_theme(),  # "shiny" preset by default — polished, not plain Bootstrap
  ...
)

Bootswatch theme (for a different visual style):

page_sidebar(
  theme = bs_theme(preset = "zephyr"),  # or "cosmo", "minty", "darkly", etc.
  ...
)

Custom colors and fonts:

page_sidebar(
  theme = bs_theme(
    version = 5,
    bg = "#FFFFFF",
    fg = "#333333",
    primary = "#2c3e50",
    base_font = font_google("Lato"),
    heading_font = font_google("Montserrat")
  ),
  ...
)

Auto-brand from
_brand.yml
: If a

_brand.yml

file exists in your app or project directory,

bs_theme()

automatically discovers and applies it. No code changes needed. Requires the

brand.yml

R package.

bs_theme(brand = FALSE)    # Disable auto-discovery
bs_theme(brand = TRUE)     # Require _brand.yml (error if not found)
bs_theme(brand = "path/to/brand.yml")  # Explicit path

Theming Workflow

Start with the
```
"shiny"
```
preset (default) or a Bootswatch theme close to your desired look
Customize main colors (
```
bg
```
,
```
fg
```
,
```
primary
```
)
Adjust fonts with
```
font_google()
```
or other font helpers
Fine-tune with Bootstrap Sass variables via
```
...
```
or
```
bs_add_variables()
```
Add custom Sass rules with
```
bs_add_rules()
```
if needed
Enable
```
thematic::thematic_shiny()
```
so plots match the theme
Use
```
bs_themer()
```
during development for interactive preview

Example:

theme <- bs_theme(preset = "minty") |>
  bs_theme_update(
    primary = "#1a9a7f",
    base_font = font_google("Lato")
  ) |>
  bs_add_rules("
    .card { box-shadow: 0 2px 8px rgba(0,0,0,0.1); }
  ")

bs_theme()

Central function for creating Bootstrap themes. Returns a

sass::sass_bundle()

object.

bs_theme(
  version = version_default(),
  preset = NULL,        # "shiny" (default for BS5+), "bootstrap", or Bootswatch name
  ...,                  # Bootstrap Sass variable overrides
  brand = NULL,         # brand.yml: NULL (auto), TRUE (require), FALSE (disable), or path
  bg = NULL, fg = NULL,
  primary = NULL, secondary = NULL,
  success = NULL, info = NULL, warning = NULL, danger = NULL,
  base_font = NULL, code_font = NULL, heading_font = NULL,
  font_scale = NULL,    # Scalar multiplier for base font size (e.g., 1.5 = 150%)
  bootswatch = NULL     # Alias for preset
)

Use

bs_theme_update(theme, ...)

to modify an existing theme. Use

is_bs_theme(x)

to test if an object is a theme.

Presets and Bootswatch

The "shiny" preset (recommended):

bs_theme()

defaults to

preset = "shiny"

for Bootstrap 5+. This is a polished, purpose-built theme designed specifically for Shiny apps — it is not plain Bootstrap. It provides professional styling with well-chosen defaults for cards, sidebars, value boxes, and other bslib components. Start here and customize with colors and fonts before reaching for a Bootswatch theme.

Vanilla Bootstrap: Use

preset = "bootstrap"

to remove the "shiny" preset and get unmodified Bootstrap 5 styling.

Built-in presets:

builtin_themes()

lists bslib's own presets.

Bootswatch themes:

bootswatch_themes()

lists all available Bootswatch themes. Choose one that fits the app's purpose and audience — don't apply one by default.

Popular options:

"zephyr"

(light, modern),

"cosmo"

(clean),

"minty"

(fresh green),

"flatly"

(flat design),

"litera"

(crisp),

"darkly"

(dark),

"cyborg"

(dark),

"simplex"

(minimalist),

"sketchy"

(hand-drawn).

Main Colors

The most influential colors — changing these affects hundreds of CSS rules via variable cascading:

Parameter	Description
`bg`	Background color
`fg`	Foreground (text) color
`primary`	Primary brand color (links, nav active states, input focus)
`secondary`	Default for action buttons
`success`	Positive/success states (typically green)
`info`	Informational content (typically blue-green)
`warning`	Warnings (typically yellow)
`danger`	Errors/destructive actions (typically red)

bs_theme(
  bg = "#202123", fg = "#B8BCC2",
  primary = "#EA80FC", secondary = "#48DAC6"
)

Color tips:

```
bg
```
/
```
fg
```
: similar hue, large luminance difference (ensure contrast for readability)
```
primary
```
: contrasts with both
```
bg
```
and
```
fg
```
; used for hyperlinks, navigation, input focus
Colors can be any format
```
htmltools::parseCssColors()
```
understands

Typography

Three font arguments:

base_font

heading_font

code_font

. Use

font_scale

to uniformly scale all font sizes (e.g.,

1.5

for 150%).

Each argument accepts a single font, a

font_collection()

, or a character vector of font names.

font_google()

Downloads and caches Google Fonts locally (

local = TRUE

by default). Internet needed only on first download.

bs_theme(
  base_font = font_google("Roboto"),
  heading_font = font_google("Montserrat"),
  code_font = font_google("Fira Code")
)

With variable weights:

font_google("Crimson Pro", wght = "200..900")

With specific weights:

font_google("Raleway", wght = c(300, 400, 700))

Recommend fallbacks to avoid Flash of Invisible Text (FOIT) on slow connections:

bs_theme(
  base_font = font_collection(
    font_google("Lato", local = FALSE),
    "Helvetica Neue", "Arial", "sans-serif"
  )
)

Font pairing resource: fontpair.co

font_link()

CSS web font interface for custom font URLs:

font_link("Crimson Pro",
  href = "https://fonts.googleapis.com/css2?family=Crimson+Pro:wght@200..900")

font_face()

For locally hosted font files with full

@font-face

control:

font_face(
  family = "Crimson Pro",
  style = "normal",
  weight = "200 900",
  src = "url(fonts/crimson-pro.woff2) format('woff2')"
)

font_collection()

Combine multiple fonts with fallback order:

font_collection(font_google("Lato"), "Helvetica Neue", "Arial", "sans-serif")

Low-Level Theming Functions

For customizations beyond

bs_theme()

's named parameters. These work directly with Bootstrap's Sass layers.

bs_add_variables()

Add or override Bootstrap Sass variable defaults:

theme <- bs_add_variables(
  bs_theme(preset = "sketchy", primary = "orange"),
  "body-bg" = "#EEEEEE",
  "font-family-base" = "monospace",
  "font-size-base" = "1.4rem",
  "btn-padding-y" = ".16rem"
)

The
.where
parameter controls placement in the Sass compilation order:

`.where`	When to use
`"defaults"` (default)	Set variable defaults with `!default` flag. Placed before Bootstrap's own defaults.
`"declarations"`	Reference other Bootstrap variables (e.g., `$secondary` ). Placed after Bootstrap's defaults.
`"rules"`	Placed after all rules. Rarely needed.

Referencing Bootstrap variables:

# This fails in bs_theme() because $secondary isn't defined yet:
# bs_theme("progress-bar-bg" = "$secondary")

# Use bs_add_variables with .where = "declarations" instead:
bs_theme() |>
  bs_add_variables("progress-bar-bg" = "$secondary", .where = "declarations")

bs_add_rules()

Add custom Sass/CSS rules that can reference Bootstrap variables and mixins:

theme <- bs_theme(primary = "#007bff") |>
  bs_add_rules("
    .custom-card {
      background: mix($bg, $primary, 95%);
      border: 1px solid $primary;
      padding: $spacer;

      @include media-breakpoint-up(md) {
        padding: $spacer * 2;
      }
    }
  ")

From external file:

bs_add_rules(sass::sass_file("www/custom.scss"))

Available Sass functions:

lighten()

darken()

mix()

rgba()

color-contrast()

. Available Bootstrap mixins:

@include media-breakpoint-up()

@include box-shadow()

@include border-radius()

bs_add_functions() and bs_add_mixins()

Add custom Sass functions or mixins to the theme bundle:

theme |>
  bs_add_functions("@function my-tint($color) { @return mix(white, $color, 20%); }") |>
  bs_add_rules(".highlight { background: my-tint($primary); }")

bs_bundle()

Append

sass::sass_bundle()

objects to a theme (for packaging reusable theme extensions):

my_extension <- sass::sass_layer(
  defaults = list("my-var" = "red !default"),
  rules = ".my-class { color: $my-var; }"
)
theme <- bs_theme() |> bs_bundle(my_extension)

Bootstrap Sass Variables

Pass any Bootstrap 5 Sass variable through

bs_theme(...)

bs_add_variables()

Finding variable names: https://rstudio.github.io/bslib/articles/bs5-variables/

Common variables:

bs_theme(
  "border-radius" = "0.5rem",
  "card-border-radius" = "1rem",
  "card-bg" = "lighten($bg, 5%)",
  "navbar-bg" = "$primary",
  "link-color" = "$primary",
  "font-size-base" = "1rem",
  "spacer" = "1rem",
  "btn-padding-y" = ".5rem",
  "btn-padding-x" = "1rem",
  "input-border-color" = "#dee2e6"
)

Values can be Sass expressions referencing variables, functions, and math.

Bootstrap CSS Custom Properties

See sass-and-css-variables.md for details on:

How Sass variables compile into
```
--bs-*
```
CSS custom properties
Runtime vs compile-time variable layers
How Bootstrap 5.3 color modes use CSS variable overrides
Per-element theming with
```
data-bs-theme
```
CSS utility classes for one-off styling

Dark Mode and Color Modes

See dark-mode.md for details on:

Bootstrap 5.3's client-side color mode system (
```
data-bs-theme
```
attribute)
```
input_dark_mode()
```
and
```
toggle_dark_mode()
```
for user-controlled switching
Server-side theme switching with
```
session$setCurrentTheme()
```
Writing custom Sass that works across light/dark modes
Component compatibility (what responds to theming, what doesn't)

Theming R Plots

bs_theme()

only affects CSS. R plot output (rendered server-side as images) won't auto-match. Use the

thematic

package:

library(thematic)
thematic_shiny(font = "auto")  # Call before shinyApp()
shinyApp(ui, server)

Works with base R, ggplot2, and lattice
Translates CSS colors into R plotting defaults
```
font = "auto"
```
also matches fonts from
```
bs_theme()
```
Complements
```
bs_themer()
```
for real-time preview

Set global ggplot2 theme for further consistency:

library(ggplot2)
theme_set(theme_minimal())

Dashboard Background Styling

The

bslib-page-dashboard

CSS class adds a light gray background behind the main content area, giving dashboard-style apps a polished look where cards stand out against the background. This is a theming detail — it doesn't change layout behavior, only the visual treatment.

For
page_sidebar()
dashboards:

page_sidebar(
  class = "bslib-page-dashboard",
  title = "My Dashboard",
  sidebar = sidebar(...),
  ...
)

For
page_navbar()
with dashboard-focused pages: Apply the class to individual

nav_panel()

containers (not

page_navbar()

itself) so only dashboard-oriented pages get the gray background:

page_navbar(
  title = "Analytics",
  nav_panel("Dashboard", class = "bslib-page-dashboard",
    layout_column_wrap(...)
  ),
  nav_panel("Report",
    # No dashboard class — standard white background for prose/reports
    ...
  )
)

Interactive Theming Tools

bs_theme_preview()

Standalone demo app for previewing a theme with many example UI components:

bslib::bs_theme_preview()                        # Default theme
bslib::bs_theme_preview(bs_theme(preset = "darkly"))  # Custom theme

Includes the theming UI by default (

with_themer = TRUE

run_with_themer()

Run an existing Shiny app with the theme editor overlay (instead of

shiny::runApp()

run_with_themer(shinyApp(ui, server))
run_with_themer("path/to/app")

bs_themer()

Add the theme editor to your own app's server function:

server <- function(input, output, session) {
  bs_themer()  # Add during development, remove for production
  # ...
}

All three tools print the resulting

bs_theme()

code to the R console for easy copy-paste. Limitations: Bootstrap 5+ only, Shiny apps and

runtime: shiny

R Markdown only, doesn't affect 3rd-party widgets that don't use

bs_dependency_defer()

Theme Inspection

Retrieve computed Sass variable values:

vars <- c("body-bg", "body-color", "primary", "border-radius")
bs_get_variables(bs_theme(), varnames = vars)
bs_get_variables(bs_theme(preset = "darkly"), varnames = vars)

Check contrast (for accessibility):

bs_get_contrast(bs_theme(), c("primary", "dark", "light"))

Aim for WCAG AA compliance: 4.5:1 for normal text, 3:1 for large text.

Best Practices

Prefer
bs_theme()
over custom CSS -- variables cascade to all related components automatically
Pin Bootstrap version:
```
bs_theme(version = 5)
```
prevents breakage if defaults change
Use fallback fonts with
```
font_collection()
```
to avoid FOIT on slow connections
Test across components: inputs, buttons, cards, navs, plots, tables, modals, toasts, mobile
Check accessibility with
```
bs_get_contrast()
```
and browser dev tools
Use CSS utility classes for one-off styling instead of custom CSS (see sass-and-css-variables.md)
Organize complex themes in a separate
```
theme.R
```
:

# theme.R
app_theme <- function() {
  bs_theme(
    version = 5,
    primary = "#2c3e50",
    base_font = font_google("Lato"),
    heading_font = font_google("Montserrat", wght = c(400, 700))
  ) |>
    bs_add_rules(sass::sass_file("www/custom.scss"))
}

Reference Files

sass-and-css-variables.md -- Bootstrap's two-layer variable system, CSS custom properties, utility classes
dark-mode.md -- Color modes, dark mode, dynamic theming, component compatibility

shiny-bslib-theming

NPX Install

Tags

SKILL.md Content