We’ve released a new online Theme Builder tool for manipulating themes. It’s best suited to making minor tweaks to existing themes. The video below is a quick walkthrough of how to load a theme from the Directory and make tweaks to it.
The theme builder is not a complete solution, by any means, but it’s a lot easier than directly editing the raw JSON format in a text editor.
Reading over the documentation on Creating Themes is still a good idea to get a feel for what the different elements in the theme do.