I suggest trying an add-on, for example, the following, although I'm sure there are many options:

https://addons.mozilla.org/firefox/addon/color-that-site/

Thanks Mate but far to complicated for such a simple thing, I am using FFox that came with Linux Mint 17.3, if that answers any questions, think I am getting to old for this, again thanks. Regards bob

The normal behavior of Firefox is to use the backgrounds specified by the website, either/both background colors or background images. You have two choices in the built-in settings:

(1) Set a background color to be used when websites do not specify the background color

(2) Override all website colors with your preferred text and background colors

You should definitely set #1, but the problem with #2 is that it makes some sites unusable when background colors and image are necessary to find buttons and other elements. That is why I suggest using an add-on instead of #2.

Here's where you experiment:

"3-bar" menu button (or Edit menu) > Preferences

In the left column, click Content, then on the right side, click Colors. Be careful not to set black on black or other low contrast combination, since that will make it hard to see anything and therefore challenging to undo the change.

To override website colors, click the button that usually says "Only with high contrast themes" and select "Always".

Please see the attached screen shot.