Reuse proper term for colour

aleksis/apps/chronos/static/css/chronos/timetable.css

@@ -81,7 +81,7 @@ table.substitutions td, table.substitutions th {
 }
 
 .lesson-with-sub {
-    border: 3px solid #ff0000;
+    border: 3px solid red;
     border-radius: 3px;
 }