[dev] Clean up the custom color registry items

[kittaakos]

Describe the problem

RC-8 could show the board selector with eight custom colors, the current state fails to do the same with 17 custom color entries.

Clean it up. Adjust other themes with the fallback.

To reproduce

This is a code quality issue.

Expected behavior

The themes work, IDE2 code does not contain unused color entries.

Arduino IDE version

7680194

Operating system

macOS

Operating system version

12.3.1

Additional context

No response

Issue checklist

• I searched for previous reports in the issue tracker
• I verified the problem still occurs when using the latest nightly build
• My report contains all necessary details