fix(theme): techdocs addon subheader bg-color (#1703)

Signed-off-by: Yi Cai <[email protected]>

Author: ciiay

workspaces/theme/.changeset/big-stingrays-heal.md

@@ -0,0 +1,5 @@
+---
+'@red-hat-developer-hub/backstage-plugin-theme': patch
+---
+
+Scope TechDocs subheader toolbar to use `background.paper`, fixing the gray strip under the page header. Adds a `MuiCssBaseline` rule targeting `[class*="BackstageHeader-header-"] + [class*="MuiToolbar-root"]` so other toolbars remain unaffected.

workspaces/theme/plugins/theme/src/utils/createComponents.ts

@@ -88,6 +88,9 @@ export const createComponents = (themeConfig: ThemeConfig): Components => {
         'pre, code': {
           fontFamily: redHatFonts.monospace,
         },
+        "[class*='BackstagePage-root-'] > [class*='MuiToolbar-root']": {
+          backgroundColor: theme.palette.background.paper,
+        },
       };
     },
   };