You may not notice any difference, but the website will now be using the colour scheme you have on your computer. If these colours are OK then you need not do anything else, other than clicking 'OK' or pressing 'Enter' on your keyboard.
If however you still need to change the text or background colours on the web page take the following steps:
On a Mac you have limited options to change your OS colour scheme so you are more likely to need to set text and background colours just for Firefox. To do this: