User tours can appear as white-on-white text

From Dan: Lets just focus on checking for regressions, unless you are particularly keen to try and find a way to reproduce the original issue, to ensure that we've not introduced regressions is enough.

Steps to reproduce: (Ryan could not reproduce this. I originally found this in a custom OU theme, and it was easy to reproduce there. I am sure this worked for me when I submitted this issue for peer review some weeks ago.).

1. Ensure you are using Clean theme.
2. Make a post in Site news with HTML souce '<p class="bugdemo" style="color: white; background-color: black;">White on dark</p>' (of, if you use a custom theme, find any content where the text-colour is white.
   1. You may need to enable it: site administration > front page > front page settings - add "Announcements" as one of the "Front page items when logged in"
3. Create a user tour, with a step targetting the .bugdemo CSS selector.
4. View the user tour.
5. Ceck that you can read the text in the tour pop-up.