In this way we can use color tints other than grays in our custom theme, w/o having to extend it later (e.g `$primary: $indigo-600;` ). This could be done in the project variables file, but i believe most developers just include a full local copy of `scss/_variables.scss` before the the original `default` file and change the values the need (maybe also removing the !default flag) so that it's easier to track changes and custom values during upgrades.
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