Inform base_root.htm template about ACCESSIBILITY_MODE

[jacobtylerwalls]

The 7.5 setting ACCESSIBILITY_MODE needs to be exposed in base_root.htm somehow so that Vue apps can know whether to use higher color contrast if they're not extending base.htm. (Or if they are, and they want to do it without querying the document for the existence of a hard-coded stylesheet name.)

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ya I'm wondering if this interaction can also be stubbed out in core at a very basic level as well, since the theme lives there now. We'll definitely use this across applications so if it can proved out in core instead, I think it'll be helpful.

Originally posted by @chrabyrd in archesproject/arches-controlled-lists#16 (comment)