Basic keyboard accessibility CSS tweaks

Mostly doubling-up :hover styles to also cover :focus, as a first step
to making the framework more keyboard-friendly.
Additionally, fixed two small markup issues in the docs/examples to
make the "Learn more" large primary button-styled links
keyboard-focusable (as without href they're treated as non-tabable
anchors).

less/close.less

@@ -11,7 +11,8 @@
   color: @black;
   text-shadow: 0 1px 0 rgba(255,255,255,1);
   .opacity(20);
-  &:hover {
+  &:hover,
+  &:focus {
     color: @black;
     text-decoration: none;
     cursor: pointer;