Pick colours, watch a mock Blender UI update in real time, then export a .xml you drop straight into Blender via Edit ▸ Preferences ▸ Themes ▸ Install…. Themes the widgets, panels, menus, every editor background and the 3D viewport. No account, nothing uploaded — it all runs in your browser.
File name becomes the theme name in Blender's Themes list.
Works on Blender 4.x. Like it? Grab more free themes in the store.