fix: Work around bslib's $default for BS3-4 compat

Author: gadenbuie

shiny/ui/_theme_brand.py

@@ -255,11 +255,6 @@ def _prepare_color_vars(
 
             brand_color_var = f"brand_color_{thm_name}"
             defaults_dict[brand_color_var] = thm_color
-            # Currently, brand.color fields are directly named after Bootstrap vars. If
-            # that changes, we'd need to use a map here. These values can't be set to
-            # `null !default` because they're used by maps in the Bootstrap mixins layer
-            # and cause errors if a color is `null` rather than non-existent.
-            defaults_dict[thm_name] = f"${brand_color_var}"
 
         brand_color_palette = brand.color.to_dict(include="palette")
 

shiny/www/py-shiny/brand/_brand-yml.scss

@@ -69,6 +69,16 @@ $brand_typography_link_weight: null !default;
 $brand_typography_link_decoration: null !default;
 
 //*-- brand.color --*//
+$primary: $brand_color_primary !default;
+$secondary: $brand_color_secondary !default;
+$tertiary: $brand_color_tertiary !default;
+$success: $brand_color_success !default;
+$info: $brand_color_info !default;
+$warning: $brand_color_warning !default;
+$danger: $brand_color_danger !default;
+$light: $brand_color_light !default;
+$dark: $brand_color_dark !default;
+
 $body-color: $brand_color_foreground !default;
 $body-bg-dark: $brand_color_foreground !default;
 $body-bg: $brand_color_background !default;

shiny/www/shared/sass/bslib/lib/bs5/scss/_variables.scss

@@ -300,7 +300,7 @@ $cyans: (
 // Semantically, $secondary is closest to BS3's 'default' theme color;
 // so use that if specified. Otherwise, use a light instead of dark gray
 // default color for $default since that's closer to bootstrap 3's default
-$default: if(variable-exists("secondary"), $secondary, $gray-300) !default;
+$default: if(variable-exists("secondary") and type-of($secondary) == color, $secondary, $gray-300) !default;
 
 // scss-docs-start theme-color-variables
 $primary:       $blue !default;