Currently in the win32 theme, a button with .flat class has a non-flat appearance, and buttons in a toolbar appear flat irrespective of their class. This MR makes them behave similar to the buttons in Adwaita (those with .flat appear flat and non-flat buttons can be present in a toolbar)
| before | after | Adwaita |
|---|---|---|
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fixes downstream issue https://gitlab.com/inkscape/inkscape/-/issues/2896