Fix webkit calendar icon color on arc-green (#15713)

Co-authored-by: zeripath <art27@cantab.net>

web_src/less/themes/theme-arc-green.less

@@ -110,6 +110,10 @@
   --color-project-board-bg: var(--color-secondary-light-2);
 }
 
+::-webkit-calendar-picker-indicator {
+  filter: invert(.8);
+}
+
 .ui.horizontal.segments > .segment {
   background-color: #383c4a;
 }