Place a .css or .qss Qt Stylesheet file in here, and hydrus will provide it as an UI stylesheet option.
The default_hydrus.qss is now used by the client to draw some custom widget colours. Do not edit it. It is prepended to any custom stylesheet that is loaded, check it out for the class names you want want to override in your own custom QSS.
I think to do this properly we'll want folders so we can include additional assets like images.
Here's some examples, there are some QSS files buried here:
https://wiki.qt.io/Gallery_of_Qt_CSS_Based_Styles
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