Looks like in Atom you can just inspect elements of the editor and change them right there. In Brackets if you want to change the font of the editor then you have to write an extension or search for the CSS file of the editor which is barely editable with Brackets because it's slows down with the somewhat bigger CSS file, syntax highlight stops at the half of the file and searching was extremely slow for me in this big file.
You can open dev tools on either of these editors, but in both cases you're not making permanent changes to the source that will stick around the next time you launch the app.
Hacking the minified source isn't going to be fun no matter what, but if you clone Brackets from git the original LESS files are very easy to modify. Or you could just write a three line extension (https://github.com/adobe/brackets/wiki/Customize-Your-Code-F...) to adjust the CSS without modifying the core at all. That flexibility is the advantage of building on the web tech stack (an advantage shared by Brackets, Light Table, and Atom alike).
When I first saw Brackets, I was surprised to see it come out of Adobe. I had no idea that they did any open-source work, especially considering their product portfolio.