There's a closed pull request on the void-packages repository

obconf: use GTK3
https://github.com/void-linux/void-packages/pull/40398

Description:
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#### Testing the changes
- I tested the changes in this PR: **NO** (I don't use openbox, so I can't really test this)

TODO: find a stable source for the libglade patch and add it to distfiles, it looks too big to include in void-packages (396kB)
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