Developing a theme plugin for Lapce is easy. Firstly open settings, and you can change colours in theme settings, and change the UI appearance in UI Settings.
Open command palette, choose the command
Export current settings to a theme file, it will generate the theme file for you.
Change the name of the theme, and save the file.
To create a theme plugin for Lapce, follow the format of this repo https://github.com/lapce/atom-one, the important files are
volt.tomland the theme file.