bext: fix command palette styles
Created by: taras-yemets
Closes https://github.com/sourcegraph/sourcegraph/issues/39495 Closes https://github.com/sourcegraph/sourcegraph/issues/33433
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Test plan
sg run bext- open any repo on GitHub and open command palette
- ensure its styles are consistent with the rest of GitHub UI
App preview:
Check out the client app preview documentation to learn more.