Awesome! How do you open themes?
I have no Idea.
thm files can be downloaded in the Themes.zip package on the Dotgrid Itch page.
They're just text, so you can edit them by opening them up with Left, or any text editor.
To install a theme, just drag it on the app window.
How can I reset the theme to the default UI?
Press menu>reset, or ctrl+delete.