Allow modules to add custom CSS to improved_theme_settings

Problem/Motivation

Some modules may require different selectors than are supported in the improved_theme_settings module. They may need this to hook into theme settings with their own styling. It's impractical to support every possible module in the improve_theme_settings module itself.

Proposed resolution

Add an info hook that is called when improved_theme_settings generates its CSS. This allows modules to query the current theme settings and generate their own selector. One such use case is an internal Open Social extension that is adopting utility based CSS.

Follow ups to this ticket could be created to move some of the very specific queries currently included in improved_theme_settings to their own modules, this cleans up the CSS when those modules are not enabled.

Remaining tasks

• Implementation Approval

User interface changes

None

API changes

hook_improved_theme_settings_add is added to allow custom modules to add CSS styles that are dependant on theme settings.

Data model changes

None