fix(rbac): background color of sidebar in dark theme

[ciiay]

RHIDP-3038: [RBAC] conditional Side bar styles are not changing for dark theme

How to test:

1. Pull this PR to your local plugins repo
2. Go to plugins/rbac and run yarn export-dynamic
3. At your local showcase directory, go to dynamic-plugins-root/ and run

pkg=../../backstage-plugins/plugins/rbac
archive=$(npm pack $pkg)
tar -xzf "$archive" && rm "$archive"
mv package $(echo $archive | sed -e 's:\.tgz$::')

4. Go to packages/backend/package.json and update rbac backend version to "@janus-idp/backstage-plugin-rbac-backend": "4.3.2" as current rbac plugin only works with the latest rbac backend version.
5. Go to showcase root directory and run yarn install and yarn start to start backstage-showcase

[openshift-ci]

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