fix(theme): techdocs addon subheader bg-color

[ciiay]

Hey, I just made a Pull Request!

For RHDHBUGS-2075

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or Updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)

Steps to test changes on RHDH

1. Pull changes from this PR to your local rhdh-plugins path
2. Update theme package version in your local rhdh/packages/app/package.json:

    "@red-hat-developer-hub/backstage-plugin-theme": "portal:<path_to_your>/rhdh-plugins/workspaces/theme/plugins/theme",

3. Run yarn install in your local RHDH root
4. Start rhdh by yarn dev

Steps to register a demo subHeader addon on techdocs

1. Add this to your <your_local>/rhdh/dynamic-plugins/wrappers/backstage-plugin-techdocs-module-addons-contrib/src/addons.tsx

const SubheaderDemoImpl = () => {
  return (
    <div
      style={{
        display: 'flex',
        gap: 8,
        alignItems: 'center',
        padding: '6px 8px',
      }}
    >
      <button
        type="button"
        onClick={() =>
          console.log('Subheader addon clicked')
        }
        style={{
          cursor: 'pointer',
          border: '1px solid var(--backstage-divider, #e0e0e0)',
          borderRadius: 6,
          padding: '6px 10px',
          background: 'var(--backstage-background, #fff)',
          fontSize: 13,
        }}
      >
        Subheader Demo
      </button>
    </div>
  );
};

export const SubheaderDemo = techdocsModuleAddonsContribPlugin.provide(
  createTechDocsAddonExtension<{}>({
    name: 'SubheaderDemo',
    location: TechDocsAddonLocations.Subheader, // 👈 the important bit
    component: SubheaderDemoImpl,
  }),
);

2. Add this to your <your_local>/rhdh/dynamic-plugins/wrappers/backstage-plugin-techdocs-module-addons-contrib/src/index.tx to export the addedSubheaderDemo

export { ReportIssue, SubheaderDemo } from "./addons";

3. Add this to your app-config.local.yaml to register the SubheaderDemo

dynamicPlugins:
  rootDirectory: dynamic-plugins-root
  frontend:
    backstage.plugin-techdocs:
    ... # default techdocs configuration
    backstage.plugin-techdocs-module-addons-contrib:
      techdocsAddons:
        - importName: SubheaderDemo

[rhdh-gh-app]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@red-hat-developer-hub/backstage-plugin-theme	workspaces/theme/plugins/theme	patch	v0.10.1

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[lokanandaprabhu]

Tested locally, working fine.

/lgtm

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