To be able to change the styling of the checkbox it is important to understand how the plug-in is handled by an interactive grid.
Checkboxes (or icons) visible to end-user look the same but in fact they are individual elements rendered in different situations by different controllers.
These elements are rendered by the plug-in metadata procedure, the plug-in item, apex.item namespace and interactive grid.
Read below to learn what classes are applied to the checkboxes (or icons) visible to end-user and when it happens.
Inspect section Demo CSS to learn how colors of the icons are overridden.