fix(preset-shiny): Set brand color CSS var in page_sidebar()

Author: gadenbuie

inst/components/scss/page_sidebar.scss

@@ -9,6 +9,7 @@ $bslib-sidebar-padding: $spacer * 1.5 !default;
 
 .bslib-page-sidebar {
   > .navbar {
+    --#{$prefix}navbar-brand-color: var(--bslib-page-sidebar-title-color);
     border-bottom: var(--bs-border-width) solid var(--bs-border-color-translucent);
     background-color: var(--bslib-page-sidebar-title-bg);
     color: var(--bslib-page-sidebar-title-color);