Hi, i like you are one of the few free godot themes! Although I'm not sure what settings I'm missing, I can't seems to get my preview look the same as your image.
Tried Display>Window>Stretch>Scale (made it big), tried theme>DefaultBaseScale (didn't change much).
Viewing post in Waste No Space GUI comments
The problem is that with the release of Godot versions, some parameters were renamed and now the engine does not parce them, I opened the ".tres" file using an editor and replaced "margin" with "textute_margin". However, there are still other parameters that need to be corrected, because in the end the theme still looks ugly.