Adding changeset

Author: jonrohan

.changeset/witty-donuts-rest.md

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+---
+'@primer/css': patch
+---
+
+Fixing the `color-mode-theme` mixin. Currently the mixin doesn't take into account when `mode="light"` and `light-theme="dark"`. This fix allows dark themes to be set as light themes and to toggle into single light mode.