Theme is lacking a QT stylesheet

The theme is lacking a QT stylesheet in order to look more like the sample screenshot provided.

As seen [here](https://github.com/texstudio-org/texstudio/issues/45#issuecomment-368301279), we can theme the rest of the UI using this.

I've made a very basic stylesheet based on [this one](https://github.com/thatlittleboy/TeXstudio-Qt-Stylesheet/blob/master/stylesheet.qss) by [@thatlittleboy](https://github.com/thatlittleboy), I'll attach it to the issue. Feel free to use it. The colors are still very rough and don't really combine that well. This makes the UI much more consistent as seen in the screenshot below.


There are some issues with his stylesheet that are mentioned in his repo, maybe we as a community can find solution for it.

[stylesheet.txt](https://github.com/dracula/texstudio/files/7121563/stylesheet.txt) - GitHub didn't allow the .qss file to be uploaded, so the extension must be changed. The file must be placed in `C:\Users\\AppData\Roaming\texstudio` in Windows.

I'll be more than glad to help with what I can!