[>=20.04] Broken tab styling in Synaptic

[lah7]

Originally reported at https://github.com/ubuntu-mate/ubuntu-mate-artwork/issues/71

Describe the problem

In Synaptic, when the tabs are enabled for the main window, the styling is broken.

Expected behaviour

This works in 18.04:

Example 2

Another example is the GTK+ Demo (from gtk-3-examples)

Here, the "Info" tab is supposed to be selected.

Which themes are affected?

• Ambiant-MATE
• Ambiant-MATE-Dark
• Radiant-MATE