Recent issues with the database impacted not only the API but also the regular thethingsnetwork.org website because they were running under the same Django project. Yesterday we decided to split the API from the website and move it to a separate project, so that any issues with the API database wouldn't impact the regular website anymore. As a result, static files that previously lived in the same directory, are now in different locations. I will update our frontend servers to serve the css files soon, but it's not my highest priority at the moment.
tl;dr: We are aware of the issue and working on a solution.