Given a website with this simple CSS: * {background-color: transparent;} body {background-color: #ffffff; color: #000000;} The font becomes unreadable under dark themes (I tested with Obsidian Coast) because the background colour of the body element is forced to that of the current Qt/KDE widget theme colour. I have observed that if the above CSS is changed to: * {background-color: transparent;} body {background-color: #ffffff!important; color: #000000;} the background colour is correctly set to the colour given in the stylesheet. However, this should not be necessary to override the background colour set by the browser. I have observed this in both Konqueror and Rekonq. This is a major limiter in the use of dark themes and breaks many websites.
I think you need to report this to whomever created the dark theme you are using because I can easily show that no such thing happens using the default KDE theme (oxygen). I have taken your 1st css excerpt and created a minimal test case that sets the background to light yellow color. When I open the test file, the background color is correctly set to light yellow in all the browsers I tested (konq+webkit, konq+html, chromium and firefox) ; so I fail to see how this could be a webkit related bug.
Created attachment 73724 [details] minimal test case
See comment #1 and the test case in comment #2.