luci-theme-material: Update brand logo/colors

I have updated the brand.png logo to the 2020 version, and I have
made a small adjustment to the CSS to reflect its slightly larger
vertical height.

I also have made small tweaks to the system theme colors to use
the brand colors as recommended on page 5 of the logo usage
guidelines document avalible at
openwrt/branding/blob/master/openwrt-styleguide.pdf

Signed-off-by: Quentin Baker <opensource@quentb.com>