It’s because each of the “colour” options has its own CSS. I got around this problem by removing the “Customise” section from the “sidebar.php”.
Then you need to log in to your FTP and copy or move the “white.css” file from “wp-content/themes/minimalist/styles” to the “wp-content/themes/minimalist” folder.
Then you need to edit the “header.php” and remove the “/styles” bit from before the “white.css” link.
Once you’ve done that, you should be able to edit the “white.css” via the appearance editor as normal.
Of course, if you want to keep all the different colour options then don’t remove the menu but copy ALL of the .css files into the other folder.
