Log out of TopCoder, and open a challenge details page (e.g. https://www.topcoder.com/challenge-details/30056183/?type=develop). When the window is wide you see such header:

Note that both JOIN and LOG IN buttons do redirect to the same login page. Correct the JOIN button to redirect to the proper endpoint page (https://accounts.topcoder.com/member/registration).
Now, check the same page in a narrow window:

You'll see that JOIN button is present, but differs in style from the wide-screen view, and is not functional, while the LOG IN button is missing. We should have here the same styled and functional buttons, as in the wide-screen view.

In case the header code is not found in this repo, then it should be here: https://github.com/appirio-tech/tc-site