Migrate Theming Service

[droshev]

Is this a bug, enhancement, or feature request?

enhancement

Briefly describe your proposal.

The current theming service is implemented as part of @fundamental-nhx/core where we have hardcoded few themes and add 2 css files per theme. The goal of this task is to use the ui5 theming instead. They will handle the theming-base-content css files for us. What will be missing is how to pass our delta css file to ui5 theming.