What the heck is this?

Details:
Monokay with colouring of brackets you’re inside of.
Font-agnostic. Yes, it’s very visible at 9pt.

This is a plugin, you may want to uninstall it

What is a plugin?

Brackets are not rendered like this out of the box.

Either a plugin has setup a region to highlight them, or the .tmTheme file has been edited to change the bracketContentsOptions key, which is no longer fully supported.

I see.
There’s no plugin.
I edited the .tmTheme to highlight brackets with colour instead of dotted bottom border.

If you say it’s no longer “fully” supported, how can I colour the brackets properly?