Here's a no-voodoo extension that does what it says and only that.
Simply modify and apply custom css styles to brackets ui...
- open help / extensions folder
- go to user / tobibeer.brackets-custom-css
- open styles.css
- use F12 (developer tools) to inspect bracket's elements
- add desired styles
- save changes
- reload brackets ui via f5
- repeat until satisfied
Default styles increase sidebar and outline font size a wee bit.
This extension will be gone when a theme-independent user stylesheet can be defined for brackets.ui.