Hide Group Background

Leaves an open tab group without the grey box Dia draws behind it, so groups look like Arc's folders. A group you've given a colour keeps it.

Turn it on

Open Dia → Settings → Patches and switch on Hide Group Background. Or run:

diatweaks enable hideTabGroupBackground

FAQ

Does it change groups I picked a colour for?

No. Only the grey a group gets when it has no colour of its own goes.