ottyl/tickets/012-merge-custom-themes-and-imported-css.md

Merge "custom themes" and "configured/imported CSS" into one concept

Custom themes and configured/imported CSS are the same thing.

Current behavior (notes)

• Imported CSS (as pasted content or file upload) is base64-encoded and stored in the settings file.
• The "name" field is not a file name — it's a human-readable name, labeled just "name" in the UI. It gets displayed alongside the other (built-in) themes.
• The built-in themes are also auto-inserted into the settings JSON.
• Imported CSS entries can be "deleted", which removes them from the settings JSON.

Acceptance criteria

• The "custom themes" and "configured/imported CSS" features are consolidated into a single mechanism.
• Duplicate UI, settings, and code paths for the two are removed.
• Existing user configuration for either is migrated or aliased so it keeps working under the unified mechanism.