vim script for editing gvimrc, if else statement

I am new to the vim scripting. I wants to know how can I use the "if else" statement to the below situation.

In my .gvimrc I have set the background as dark and it can be change to light also

set background dark

Now I am planning to change the marginal color (LineNr) depends on the background selection like below

if background == dark
   highlight LineNr guibg=Black guifg=Yellow
else
  highlight LineNr guibg=White guifg=Blue
endif

The above statement is wrong. Please let me know how can I write the if statement for the above case