Export useful CSS constants for modules to import

[Cloud7050]

Modules usually spawn tabs, which contain their own defined elements and styling. Ideally, their design would be consistent with the rest of the frontend.

However, the frontend does not provide modules with CSS constants to import and reuse, such as for defining a consistent amount of spacing, sharing the same Blueprint border radius, or reusing an existing Ace editor theme colour.

A current way of working around this is to inspect the frontend webpage and extract relevant strings for reuse, storing them in the modules repo itself. These are duplicate values that need to be manually updated (eg when we updated to BlueprintJS V4), akin to copy pasting the js-slang List library into various modules (we now have the technology to not require doing so).

This enhancement entails exporting some useful CSS constants from the frontend such that modules are able to import them.

(This is a followup to a review comment in source-academy/modules#234.)