UI Theme Stacking

COmanage support custom UI themes. The general rule is that if we choose to apply a custom UI theme, then it will override the default CSS classes and will replace any other custom theme.
For example if i apply a custom UI theme at CO level and then a different one for an Enrollment Flow, then during the Enrollment flow the CO level UI theme will not be loaded.

What we propose is instead of replacing the UI custom themes to stack them. This will result in a hierarchy of themes where the leaf will override its parent.
Since this is not always the wanted behaviour we also propose the feature to be configurable. There should be a CO level configuration which will enable or disable stacking the custom UI on top of the Platform's custom UI, if any. Then there will be an Enrollment Flow level configuration which will enable or disable stacking the custom UI on top of CO's custom UI.
Each level will be able to override only the previous one.