[Bug] Fixing flash of unstyled dashboard content (#25559)

Fixes #25375

Author: cchaos

src/core_plugins/kibana/public/dashboard/_hacks.scss

@@ -24,8 +24,8 @@ dashboard-listing {
 // Yes, this is a hack because bootstrap will be removed and everything must move to EUI theming
 // Can't reliably remove all/any of these because of embeddables
 
-.tab-dashboard.theme-dark {
-  @import './../../../../../node_modules/@elastic/eui/src/themes/k6/k6_colors_dark';
+.theme-dark {
+  @import '@elastic/eui/src/themes/k6/k6_colors_dark';
 
   // /src/ui/public/styles/bootstrap/scaffolding.less
   a {

src/core_plugins/kibana/public/dashboard/_index.scss

@@ -16,14 +16,14 @@
 /**
  * 1. Don't duplicate styles in dark mode
  */
-.tab-dashboard.theme-light { /* 1 */
+.theme-light { /* 1 */
   @import 'dashboard_app';
   @import 'grid/index';
   @import 'panel/index';
   @import 'viewport/index';
 }
 
-// Imports outside of .tab-dashboard selector don't change between light/dark modes
+// Imports outside of theme selector don't change between light/dark modes
 @import 'components/index';
 
 // Must be outside of theme selector because it lives in a portal
@@ -33,14 +33,16 @@
 // EUI global scope -- dark
 @import '@elastic/eui/src/themes/k6/k6_colors_dark';
 
-.tab-dashboard.theme-dark {
+.theme-dark {
   background-color: $euiColorEmptyShade;
 
   @import 'dashboard_app';
   @import 'grid/index';
   @import 'panel/index';
   @import 'viewport/index';
 
-  // Vis imports
+  // Vis imports -- will have some duplicate styling
+  // because they will be imported via ui/public as well
+  // (without .theme-[] prefix)
   @import 'src/ui/public/vis/map/index';
 }