# 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.