Draft: Sort out theme selection on mac/win

Mike Kowalskirequested to merge
mkov/inkscape:update-theme-selection into master

There are no "System" UI themes in Inkscape running under macOS/win. There's no theme "Adwaita" either (it is reported as "Default" instead). We need to tailor theme selection to those platforms.

Bundled gtk theme resources (in macOS/Win builds) have undergone changes in gtk4.20. There's only a single "Default" theme with high-contrast and dark variants packed inside and selected through media queries. That impacts how we detect dark theme support (we don't).

Edited by Mike Kowalski

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