How to apply desktop theme to VLC, Virtualbox etc.?

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Welcome to the forum!

Thank you :slightly_smiling_face:
I actually wanted to write that myself but was already so tired and annoyed by my problem that I forgot to do it.

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VLC and VirtualBox are Qt-based applications, while XFCE is based upon the GTK widget toolkit.

Ah, I see, this makes things a lot clearer!

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Make sure that you have qt5ct and kvantum-qt5 installed ─ they’re both in the main repository if they’re not on your system already ─and then you can tweak the look & feel of Qt-based applications.
kvantum-qt5 ─ listed in your menu as Kvantum Manager ─ is a theme engine based upon scalable vector graphics, in which you can load special Kvantum themes. There are several such themes in the repository, such as kvantum-matcha , and others can be downloaded from store.kdeorg.

Alright, coming from Xubuntu originally I wasn’t really aware of what I can do with Kvantum, so I watched two or three tutorials. Though my theme did not show up in Kvantum I somehow managed to apply it to all the programs I tested so far.

So thank you for your useful hint!

Oh, one more question since I couldn’t answer it so far: Is it advisable to use KDE Kvantum themes with XFCE?