I've found the best way to work is my having a copy of the core.css & custom.css on my own computer & then take the source code off the dynamic home page.

Change the css links at the top of the page to match your computer & then you can work on the custom.css by editing both it and the home page source code off-line. Obviously this may then bring up some issues for other pages but it is a way to get the main work done.

Suggestion:
The custom.css file is renamed templateset.css . and called by
<link type="text/css" rel="stylesheet" media="screen" href="<%config.db_static_url%>/<%t%>.css" />

with the core.css being called by
<link type="text/css" rel="stylesheet" media="screen" href="<%config.db_static_url%>/core.css" />

This would make it less likely that the wrong custom style sheet would be edited for each template set. All style sheets could be stored in the one directory.