Bugfix for Impress Issue 457

https://github.com/impresspages/ImpressPages/issues/457

### Bug Description
ImpressPages' Plugin "TinyMCEFull" doesn't seem to work. The TinyMCE menubar didn't show

### Bugfix
The solution was actually only a CSS issue. The menu bar text had the same color as the background. Since ImpressPages uses it's own TinyMCE skin, i updated the skin files there.