Sass variables referencing CSS variables

[BSFishy]

This is an issue to document a discussion that the OUI team had about moving over to Sass variables.

When #814 was raised, I made a comment (https://github.com/opensearch-project/oui/pull/814/files#r1234380915). Essentially, my idea was for existing Sass variables to be set to CSS variables (i.e. $ouiFontFamily would resolve to var(--oui-font-family)), then introduce another variable for themes to override.

I created a POC here: BSFishy@595ebbd

This would be nice, as the point of switching to CSS variables means these values can be changed on the front end without needing to rebuild the entire library. It would mean anywhere that these variables are used would also be using CSS variables, so we wouldn't need to manually go in and change those Sass variables to CSS variables.

Essentially, we can't go this route because it would be a breaking change. Existing code expects these Sass variables to be a certain value, for example a color. If they suddenly aren't a color, the code would break.

Again, just to document the discussion we had. No action items are associated with this issue.

[joshuarrrr]

Thanks Matt!