style: change css color functions to use modern notation

- `rgba()` → `rgb()`

.stylelintrc.json

@@ -8,8 +8,6 @@
     "property-no-vendor-prefix": null,
     "selector-no-vendor-prefix": null,
     "value-no-vendor-prefix": null,
-    "color-function-notation": "legacy",
-    "alpha-value-notation": "number",
     "selector-not-notation": "simple",
     "color-hex-length": "long",
     "declaration-block-single-line-max-declarations": 3,