fix(ui): Improve light/dark theme selector label

[dnrce]

Under Settings > Appearance, update the theme selector label from "Dark Theme" to "Theme."

Previously this field was a toggle for whether "Dark Theme" was on or off, so the label "Dark Theme" was clear. Once the "auto" theme was added and this was made a selector with values "Light"/"Dark"/"Auto", it was possible to set "Dark Theme = Light" which no longer made sense. This updates the label to simply "Theme," to fit the selector.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

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[nitishfy]

LGTM!

[reggie-k]

LGTM