It's simplest to use an add-on or custom style rule.

For example, I use this rule, which replaces the white background behind the page with a slate bluegray color:

@-moz-document url(chrome://browser/content/browser.xul) {
  browser[type="content-primary"] {background: #778899 !important}
}

To install that rule (or another custom style rule) into Firefox, you can use either:

• The Stylish extension (good for quick edits and experiments)
  • Stylish: https://addons.mozilla.org/firefox/addon/stylish/
  • User style: https://userstyles.org/styles/132844/soften-the-white-flash-back-color-while-loading

• A userChrome.css file (if you have more extensions than you want already)
  • http://kb.mozillazine.org/UserChrome.css

It doesn't work, I added "5CACEE" instead of that number (in the window where that code already was), but still white background...

The above style rule covers the time before Firefox starts painting the default page background color (or the page's own background color). To soften that a bit you could change the default page background from pure white to light gray:

(0) Select and copy the following color value:

#F8F8F8

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste color and pause while the list is filtered

(3) Double-click the browser.display.background_color preference, delete the existing value, and paste the value for light gray

This color will show on pages that do not specify a background color, such as

https://www.jeffersonscher.com/res/jstest.php

Found the bug, I had "use system color" ticked in the firefox color settings... disabled, now working as expected! :D (And following your steps!)