hterm: add a helper for setting CSS variables
hterm: add a helper for setting CSS variables

This makes the code a bit easier to read and encourages us to use
CSS vars even more!

Change-Id: I10f677b638aef6daa58b6557a864397d60a15598
Reviewed-on: https://chromium-review.googlesource.com/602792
Reviewed-by: Brandon Gilmore <adbfbaf7e4f927b45ce8ad3e5043a632414a833b@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm