I would love for it to stay as simple and barebones as it is and not accrete crufty features such as applying the color theme to vacuous stuff like a generic layout on generic text/code, that is not representative of where I will use it anyway, and it won't know which color I'll use for what.

    fswatch ~/Downloads "mv ~/Downloads/colorscheme.less /somewhere/else/with/assets && firefox -remote \"openURL(127.0.0.1:3000/whatever, new-tab)\""

Bam. My layout, my content.

[0]: https://github.com/alandipert/fswatch.git

[1]: replace fswatch with inotifywait on linux

[2]: I actually use shell-wrapped AppleScript to remote control Chrome from the command line.

[3]: also rb-inotify, rb-fsevent, rb-fchange gems (the last one is for Windows)