Consolidate recent theme change docs in THEMES.md

[naseschwarz]

This pull request consolidates #2570, #2214, #2564 (via #2573) into a single pull. The result from just merging seemed random, this adds a bit more structure to the document.

It changes the following:

• Add to THEME.md:
  • Custom syntax highlighting
    • Using default syntect themes (picked form Add doc for syntax in THEMES.md #2570)
    • Using custom TextMate themes (for Update THEMES.md regarding custom syntax themes #2573)
  • Reference themes (picked from add catppuccin preset themes repo in THEMES.md #2214)
• Enables syntax highlighting for RON data in THEME.md

I followed the checklist:

• Checked the THEMES.md rendering on github.
• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

[naseschwarz]

@VasilisManol: I picked your change from #2570 and extended it about the changes for #2564 to streamline merging this.

@acuteenvy, @VasilisManol: If you could find the time, I'd appreciate any feedback you might have.