feat: add light/dark mode media query mixin

src/scss/_mixins.scss

@@ -1,20 +1,30 @@
 @mixin mq-bigscreen() {
   @media (min-width: 125em) {
-    @content
+    @content;
   }
-};
+}
 
 @mixin mq-desktop() {
   @media (min-width: 50em) {
-    @content
+    @content;
   }
-};
+}
 
 @mixin mq-mobile() {
   @media (max-width: 35em) {
-    @content
+    @content;
   }
-};
+}
+
+@mixin theme($theme) {
+  @media (prefers-color-scheme: $theme) {
+    @content;
+  }
+}
+
+@mixin radial-background($primary, $secondary) {
+  background: radial-gradient(circle at top left, $primary 5%, $secondary 100%) fixed no-repeat;
+}
 
 @mixin button($btn-color, $darken: 10%) {
   box-shadow: 0 0.5em 0 0 darken($btn-color, $darken);