dhcp.cgi: Update to naming of variables in css section

dhcp.cgi: Update to naming of variables in css section

- After looking at other .cgi files I realised that I had named the variables based on
   their dhcp usage only. However colouryellow or color20 etc can be used not only in a
   row but also in a cell or a column. Rather than ending up with multiple copies of the
   same colour for rows, cells, columns etc I have simplified the variable names to just
   the colour. They can then be used in whatever situation is wanted. The rest of the cgi
   code will give the view of what structure the colour is being applied to.
- This will limit the number of different css variables defined in the header.pl file

Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>