luci-theme-openwrt-2020: honor CSS definitions passed by views
Legacy Lua views might declare a global `css` variable to inject custom
styles into the template header. Support this mechanism in the OpenWrt 2020
theme as well.
Ref: https://github.com/openwrt/luci/issues/4972
Signed-off-by: Jo-Philipp Wich <jo@mein.io>