This shouldn't be too hard, because GTK+ 4 uses CSS. The selectors are different and the default settings are different.
The GTK+ 3 themes are a good starting point: there are some errors, some widgets have weird backgrounds, some images are stretched, some widgets use the Adwaita style ... but it looks like 80% of the CSS can be reused:
dimkr
commented
2 years ago 
This shouldn't be too hard, because GTK+ 4 uses CSS. The selectors are different and the default settings are different.
The GTK+ 3 themes are a good starting point: there are some errors, some widgets have weird backgrounds, some images are stretched, some widgets use the Adwaita style ... but it looks like 80% of the CSS can be reused: