At the moment, there’s only ever one active theme, and it’s in this file:
So, to “select” a different theme, you’d overwrite the contents of that file with the new theme. It’s not ideal, and @jessegrosjean is working on a better system.
What I do is keep my themes in that folder, with the theme’s name as a suffix. So, I have “theme.less.matt”, and “theme.less.solarized-dark”, and so on. When I want to switch themes, I do the appropriate overwrite in the Terminal, e.g.
cd ~/Library/Application Support/TaskPaper/
cp theme.less.solarized-dark theme.less
TaskPaper’s UI will update live if it’s open while you edit (or overwrite) themes.