There is a custom syntax color theme maker for Visual Studio here: http://studiostyl.es/schemes/create. Its user interface is pretty close to what you’re describing. And there might be some way to convert Visual Studio color themes for use with other editors.
However, that page is not very good for creating dark-background themes. It’s missing a bunch of styles that I saw fairly often in C# code, and their default light colors look awful on a dark background.
i would love to see this applied in a sort of CSS zen garden way where a simple layout's color scheme is updated continuously based off of moving your mouse around.
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)\""
[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)
That said, I would love a page that had some ipso-lorem type code C, Perl, js, C++, Java, Etc where each click would change the 'type' of element (keyword, comment, etc) and you could mouse around to set the color. Then right click and download the vim color scheme.