The ability to create your own themable CSS has been present in SharePoint for a long time:
How to: Make custom CSS files themable in SharePoint 2013
Working with the SharePoint Theming Engine
You can even define custom names/properties in your .spcolor files which can be used in your custom CSS. My colleague Matt Holden has a great post on this here:
Custom .spcolor files for SharePoint Composed Looks
Now, the SharePoint theming engine uses CSS comments to determine the color replacement:
As per Matt's post, the theming engine replaces the values at run-time to the color defined in the ContentAccess1 section in the spcolor file.
For the theming engine to successfully carry out this replacement, the ReplaceColor comment needs to be present in the CSS file we load in SharePoint.
When I minified the Themable.css file using the clean-css gulp plugin, it stripped out all the comments which made the replacement not work:
Then I noticed that the clean-css plugin provides an option to keepSpecialComments when it does the minification. I tired using that but it still did not work as special comments have to start with an exclamation mark: https://github.com/jakubpawlowicz/clean-css#how-to-preserve-a-comment-block
Adding an exclamation mark before the comment was not going to work as that would be invalid syntax for the SharePoint theming engine.
So my final solution was to convert the ReplaceColor comment to a special comment before processing it through the minifier, then letting the minifier do it's thing by minifying the css file but preserving the special comments, and then after the minification, again converting the special comment to a regular comment suitable for the SharePoint theming engine:
Which then gave me the minified CSS along with the required comments preserved:
Thanks for reading!