Each style has field "version" in style.cfg
Prosilver and subsilver2 use it to display phpBB version, customized styles use it to display style version that is often different from phpBB version.
To make styles more compatible with extensions, they should use the same metadata format: composer.json.
I suggest to split it into 2 fields:
New suggestion: Use composer.json's require option to list a particular phpbb version that is required for the style to work. Use a custom composer package type (phpbb3-style since we use phpbb3-extension) for styles.
If phpBB version is known, it makes it easier to track compatibility, allowing to warn user that style is outdated when he tries to install it and show list of outdated styles to user.
Patch for change to composer.json
(Merged) Patch for simple change to .cfg file