Your theming question would be better in the theme forum, so I've moved it there. Please do try to post in the appropriate forum.
In the sticky
How to use a REAPER theme you'll see a section on custom colouring, with a link to a more in-depth explanation of the difference between the legacy custom colouring and post-V5 custom colouring methods that a themer might choose to use.
TL/DR : that preference is greyed out when you're using a modern theme to indicate that the theme doesn't make use of that preference, it does something different/better.
If you really want to break things:
You could comment out the 'tinttcp' line in the rtconfig to re-enable the legacy tinting controls in the preferences when you're using that theme, but since the theme uses the modern colouring method instead, you're not going to like the results.
You could comment out the 'version 5' line in the rtconfig to force the theme to use the legacy colouring method, if you have a burning desire to make the theme look awful.