The Style Switcher module takes the fuss out of creating themes or building sites with alternate stylesheets. Themer can provide a theme with alternate stylesheets. Site builder can add alternate stylesheets found in the internet right in the admin section. And this module presents all those styles as a block with a list of links. So any site user is able to choose the style of the site he/she prefers.
Settings can be added to your settings page the usual way. If you declare the Style (CSS) Settings module only as a soft-dependency (thus not required), it makes sense to show settings only if the Style Settings module is enabled by wrapping all Style Settings specific form elements inside a conditional: