Create option to add a supplemental stylesheet to Basis - to safely make css updates

Description of the need

We are discussing the best way to add small css changes to Basis in core without breaking backward compatibility in https://github.com/backdrop/backdrop-issues/issues/4167

Well, there is no consensus yet on a specific solution, I'm opening this ticket to start work on what appears (to me) to be the leading contender for a solution. Partially, because I believe that this option might be relatively easy to test and once we have a PR we'll be in a better position to evaluate this option.

Proposed solution

With each minor release for BackdropCMS (as needed), we would have the option to add a supplemental stylesheet in which we can add or override existing CSS. Each supplemental stylesheet would have a checkbox to indicate that it should be loaded. The checkbox for new sites would automatically be set to include all supplemental stylesheets, but on existing sites the site admin would have to manually choose to add supplemental stylesheets.

Some sample code has been posted here (may or may not be helpful): https://github.com/backdrop/backdrop-issues/issues/4167#issuecomment-548...

I assume the supplemental stylesheet button would go on the settings page for the Basis theme. I think that we may need to redesign that page to accommodate this setting (we can come back to that later).

Alternatives that have been considered

A number of alternatives have been listed in the issue summary for the following issue: https://github.com/backdrop/backdrop-issues/issues/4167

Other information

• We could try to only include supplemental stylesheets on alternate releases (unless absolute necessary).
• I assume that all these css changes would get added to default stylesheets on next major release.

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PR by @stpaultim that has a UI setting https://github.com/backdrop/backdrop/pull/3224 PR by @jenlampton that loads multiple stylesheets https://github.com/backdrop/backdrop/pull/3243