luci-theme-material: partial style overhaul

* Restore theme to near original style
* Properly declare variable fallbacks
* More consistent table and border styling
* Improve responsive design scaling
* Add static ripple effect to sidebar menu
* Style command output as terminal
* Comb coding style and widespread cleanup
* Various other improvements and fixes

Signed-off-by: Shun Jou <shunjou@gmail.com>