Your theme is LOVELY and immediate buy+follow from me. I hope to see more themes from you! You should really have an HTML5 demo project here so people can see it in action!
BTW, if into your tres files and replace:
[ext_resource path="res://pixel_ui_theme/m5x7.ttf" type="DynamicFontData" id=3]
– with –
[ext_resource path="./m5x7.ttf" type="DynamicFontData" id=3]
Then people won’t need to put the theme in a particular path.
(and replace res://pixel_ui_theme with . in all of your tres files)