UI: When mouse hovers over button, make bg color just slightly darker

Previously, the color would get so dark it would blend with the page background
and the button text color. I'm not sure I picked the best color; just picked
something that works.

templates/default-layout.cassius

@@ -76,6 +76,9 @@ input[type="submit"]
     border:        none
     border-radius: 0.618em
 
+input:hover[type="submit"]
+    background:  #{dark gray}
+
 .instance
     color: #{light blue}
     font:  180% monospace