How to make Themes for RPCS3?

qss are basically CSS files for QT.

You just have to know CSS and QT stylesheet properties, and use any text editor like Notepad++ to write a CSS sheet.

Thx for the Answer! :-)

But what I had in mind was more like a design-programm for windows similar to Visual Studio or sketch on OSX.
My plan is to use .jpg's to style the Buttons. For example to change the style from now(VLC Player-like) to, lets say Windows Media Player-like(a collored UI which looks more plastic than just gray Buttons).

So my plan is to keep the layout as it is, just edit the looks in a way everythings fit together and give it a more plastic design. For example, PS3-style color and Text.

Is it even possible? I searched the whole internet the last 3 days, but I found nothing realy useful. Or is editing with Notepad++ the only option to style the UI?