Umbraco skin for TinyMCE

[bjarnef]

Is your feature request related to a problem? Please describe.
Currently the TinyMCE richtext editor use the default skin, but maybe we should have a custom Umbraco skin instead, e.g. to customize button colors in dialogs.

Describe the solution you'd like
A custom skin for TinyMCE editor: https://www.tiny.cloud/docs/advanced/creating-a-skin/