On the Brainstorm site a lot of text is set to the default system colour, but the backgrounds are coded to be light colours like white. For people who have light text by default (eg. white-on-black themes) this can make the site unreadable without selecting all of the text.
Looking at the OP's screenshot, this the same problem I'm having, but on a different portion of the web page. My problem has been the "Post your comment" text entry box. I have been extremely frustrated trying to enter comments on this site because it does not anticipate my dark theme. I have tried switching Firefox's theme back to default (I use PitchDark), and that did not help. The difference comes from the control set used in the Gnome Theme.
My GTK/Gnome theme was aud-Default. It does not allow you to customize the colors. The result here was white text on a nearly-white grey background. I simply could not read the text without highlighting it, and found it so frustrating that I took to using GEdit and copy-pasting my commentary.
Almost none of the extra-dark GTK control themes seem to have good results. I was lucky enough to find MidnightOSX controls theme that shows a readablelight blue on this comment box. It will work for me, for now. But I am not sure that I am satisfied with it (outside of the joy in seeing my text as I type here now).