Go to Preferences ("Options" on Windows), select the "Content" section, click the "Colors..." button, and change the "Override the colors specified by the page with your selections above" dropdown to "Always".
In current nightly:
Go to Preferences/Options, select the "General" section, scroll down to the "Colors..." button in the "Fonts & Colors" part, then as above.
Before Firefox 37 it was just a "Allow pages to choose their own colors" checkbox (which you could uncheck). _That_ UI goes back to at least Firefox 3.5. Likely earlier (I'm pretty sure it was in 1.0 as well), but I don't have anything older than that on hand at the moment.
On a separate note, I assume your "having sound" thing refers specifically to Linux and even more specifically to Linux systems without PulseAudio, right? Characterizing that as a blanket "dropped support for having sound" is at bit disingenuous.
but I also changed the below in about:config
* browser.display.background_color
* browser.display.foreground_color
I just find it faster to use keyboard instead of mouse :D