dhcp.cgi: Fix for bug#10629 - update bgcolor to css

dhcp.cgi: Fix for bug#10629 - update bgcolor to css

- This v2 version moves the extraHead variable to header.pl as many of the css values
   will be used in many other WUI cgi pages so makes sense to not define anew in every
   location using the bgcolor or other colour variables.
- I will submit patches to follow the same approach in all other WUI cgi pages once this
   has been submitted into next
- bgcolor was deprecated in HTML 4.01 and is not supported by HTML 5
- The orange colour for IP's that are outside the IPFire green and blue subnets does not
   work on any browser I am using.
- I used the CSS approach that @Leo used in the Zone Configuration cgi page
- This patch changes all existing bgcolor entries to the css based background-color
- Tested on my vm testbed and confirmed to work. The Orange colour for IP's outside of
   the subnet now shows up.

Fixes: Bug#10629
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Bernhard Bitsch <bbitsch@ipfire.org>
Tested-by: Bernhard Bitsch <bbitsch@ipfire.org>