@dotted I recommend not generating a theme based on the editor theme, as all icons will be included within the theme resource. However, you most likely won't use most of those icons (and those that you do use should be saved as external images).
Instead, creating a theme from scratch will give you something much more optimized.
Godot 3.4 has a revamped theme editor which may be able to generate more optimized themes from the editor, but you have to regenerate it instead of using an old theme resource generated from Godot 3.3.x.