Added new site-defined vars for tag colors

src/styles/booru-colors.scss

@@ -0,0 +1,4 @@
+// These variables are defined dynamically based on the category of the tag
+$resolved-tag-background: var(--tag-background);
+$resolved-tag-border: var(--tag-border);
+$resolved-tag-color: var(--tag-color);