Add @mixin to _taxonomies_dark.scss (#580)

Otherwise, tags have a dark background in the light theme.

assets/scss/_taxonomies_dark.scss

@@ -1,4 +1,4 @@
-.taxonomy {
+@mixin taxonomy {
   .taxonomy-element {
     background-color: $alt-bg-color-dark;
 

resources/_gen/assets/scss/scss/coder-dark.scss_1fcd9040f1144c65015c77e7b93bc5ac.json

@@ -1 +1 @@
-{"Target":"css/coder-dark.min.f37febc669ce189201c1918fac1948a254686c8d366a312c2d72b2bb71ad97d1.css","MediaType":"text/css","Data":{"Integrity":"sha256-83/rxmnOGJIBwZGPrBlIolRobI02ajEsLXKyu3Gtl9E="}}
+{"Target":"css/coder-dark.min.d8f89ef09509afb63b9b2595ee174e53cc51ce02a6f6a2179e1621f9389e4340.css","MediaType":"text/css","Data":{"Integrity":"sha256-2Pie8JUJr7Y7myWV7hdOU8xRzgKm9qIXnhYh+TieQ0A="}}