Custom CSS (#3459)

* New feature: Custom CSS file

Closes #3457

* Optimize FileBrowser file type filtering

* Show folders before files in FileBrowser

* Fix fileTypes default value

* fix filebrowser key

* Some env-vars, disable codecov/patch for now

* Start testing browser.py

* Fix env-vars

* codecov/patch still wasn't disabled

* Unnecessary statement

* Fix sorting of browse dialog items