The theme menu and how to get on it
As of today there is a new Settings page with a theme menu at the top of it. If you want your stylesheets to appear in this menu, you need to go to the stylesheet page and check the 'Include in theme menu' checkbox. This checkbox will not be shown if your stylesheet still needs updating to adapt it to the no longer terribly recent changes in the site's html. Don't forget: your users are getting an automatically updated version anyway.
The softlink color gradient
Directly below the menu are two options: 'Show the softlink color gradient' ... 'using colors from my theme'. If users check the first and uncheck the second, they continue to get the inline background colours. If you style #softlinks td, they won't see them. That would be unfriendly of you.
If they check both options, then the softlink tds are given ids from sl1 to sl64 and the colours come from rules added to your stylesheet when it is served. Any left-over cells then get the class slend. The starting colour for the gradient will be taken from the first rule found in your stylesheet for #sl1, #mainbody, or body, and the final colour from the first rule for #sl64, .slend, or .oddrow. Only colours defined in #xxx or #xxxxxx format will be recognised. Feel free to improve this if you really need to use fancy colour names or rgb(). If no starting color is found, white will be used; the default final colour is a mid grey.
A non-exhaustive test suggests the gradient already works well in most stylesheets. If anyone feels the need to style #sl1 to #sl64 individually, let me know.