Separate civicrm.css into admin and front-facing styles

Problem: civicrm.css currently contains all of Civi's styles. These styles include a lot of styling of front-end forms (things like font sizes, button sizes, input fields, etc.). Because of specificity Civi uses – often using the #crm-container ID or using two classes like .crm-container.crm-public – these styles override many modular designs the site might otherwise be using.

When a comprehensive frontend design has already been developed that includes styles for things like typography and form fields, it would be desirable to disable Civi's styles entirely. Currently, however the only setting for this is "Disable CiviCRM css" in (somewhat mysteriously) "Resource URLs." Disabling CSS here also turns off the back-end/admin CSS, meaning a designer would have to spend considerable time working on the Civi administration pages.

Consequently, I've had to spend a lot of time on front-facing Civi pages like signup forms, donation forms, and event registration un-setting Civi's styles that conflict with the site's design. That's obviously not ideal.

Civi styles shouldn't be all or nothing. Smart defaults for front-facing pages are fine, but they should be able to be easily turned off without affecting administration pages.