build: add dark theme

Author: mrholek

package.json

@@ -25,6 +25,7 @@
     "css-lint-vars": "fusv scss/ docs/assets/scss/",
     "css-minify": "npm-run-all --parallel css-minify-*",
     "css-minify-main": "cleancss -O1 --format breakWith=lf --with-rebase --source-map --source-map-inline-sources --output dist/css/ --batch --batch-suffix \".min\" \"dist/css/*.css\" \"!dist/css/*.min.css\" \"!dist/css/*rtl*.css\"",
+    "css-minify-themes": "cleancss -O1 --format breakWith=lf --with-rebase --source-map --source-map-inline-sources --output dist/css/themes --batch --batch-suffix \".min\" \"dist/css/themes/*.css\" \"!dist/css/themes/*.min.css\" \"!dist/css/themes/*rtl*.css\"",
     "css-minify-rtl": "cleancss -O1 --format breakWith=lf --with-rebase --source-map --source-map-inline-sources --output dist/css/ --batch --batch-suffix \".min\" \"dist/css/*rtl.css\" \"!dist/css/*.min.css\"",
     "css-prefix": "npm-run-all --parallel css-prefix-*",
     "css-prefix-main": "postcss --config build/postcss.config.js --replace \"dist/css/*.css\" \"!dist/css/*.rtl*.css\" \"!dist/css/*.min.css\"",