Add compact/comfortable utility mixins

scss/_zotero.scss

@@ -3,6 +3,7 @@
 
 @import "abstracts/variables";
 @import "abstracts/functions";
+@import "abstracts/mixins";
 @import "abstracts/placeholders";
 @import "abstracts/utilities";
 @import "abstracts/split-button";

scss/abstracts/_mixins.scss

@@ -0,0 +1,33 @@
+//
+// Mixins
+// --------------------------------------------------
+
+@mixin compact {
+	$selector: &;
+	@at-root [zoteroUIDensity="compact"] {
+		@if $selector {
+			#{$selector} {
+				@content;
+			}
+		}
+		
+		@else {
+			@content;
+		}
+	}
+}
+
+@mixin comfortable {
+	$selector: &;
+	@at-root [zoteroUIDensity="comfortable"] {
+		@if $selector {
+			#{$selector} {
+				@content;
+			}
+		}
+
+		@else {
+			@content;
+		}
+	}
+}