slightly lighten the danger button background colour

So it's different to the danger alert background.

Author: christianp

numbas_lti/static/variables.css

@@ -62,7 +62,7 @@
     --warning-button-active-bg: #d49917;
     --warning-button-color: black;
     --warning-button-border-color: #e2a62e;
-    --danger-button-bg: #be0000;
+    --danger-button-bg: #df0000;
     --danger-button-hover-bg: #da3125;
     --danger-button-active-bg: #f44d3d;
     --danger-button-color: white;