luci-theme-material: add back removed color variables

Partially reverts 2bdabf14c30320ef0433828b1a080f6b8fe9c430 by adding back the deleted color variables as the custom.css was used by users to override the theme with their own colors. To maintain backwards compatibility with users we should not remove/rename these as it will result in colors being applied incorrectly.

Signed-off-by: Daniel Nilsson <daniel.nilsson94@outlook.com>