Did you verify your video card or driver is working. Your descriptions sounds like your system has issues.

Make sure you haven't enabled High Contrast mode in the OS settings.

Make sure you allow pages to choose their own colors as this affects background images.

• Settings -> General -> Language and Appearance -> Colors -> Manage Colors
  "Override the colors specified by the page with your selections above"

Try "Never" if the default "Only with High Contrast themes" isn't working.

• https://support.mozilla.org/en-US/kb/disable-high-contrast-mode-firefox