Use high-contrast mode if gin classic toolbar and environment indicator enabled

Problem/Motivation

If gin classic toolbar and environment indicator are both in use, the toolbar background color is unpredictable (since it can be changed by environment indicator). In this case, the semi-transparent background color of the moderation sidebar tab can result in too little contrast in the default scheme

Proposed resolution

Update CSS to use high-contrast scheme if classic toolbar and environment indicator are is in use.

There's currently no class to signify that Environment Indicator is active if it's toolbar integration is enabled. So, we'll just use high contrast anytime the classic toolbar is enabled.

Classic toolbar sets a gin--classic-toolbar body class, but AFAICT, Environment Indicator's body class isn't working: #3320061: Add class to body when indicator is turned on.