Icons: add a CSS variable to define the icon font-size. See #75769

Author: Stefanie Hein

src/scss/themes/fylr/mixins/_icons.scss

@@ -2,7 +2,7 @@
 	content: $code;
 	display: inline-block;
 	// stylelint-disable-next-line
-	font: normal normal normal #{$font-size} 'FontAwesome';
+	font: normal normal normal var(--icon-font-size, #{$font-size}) 'FontAwesome';
 	text-rendering: auto;
 	-webkit-font-smoothing: antialiased;
 	-moz-osx-font-smoothing: grayscale;