Add css changes from MIT's local media files

While cleaning up MIT's media setup, we found changes to
onsite_ajax_status.css, which seem like they would be useful to other
chapters.  They mainly make text near the top more readable.

(cherry picked from commit 6404d0c0b2daf738f3339de13f90f031750291e6)
Merge pull request #1422 from learning-unlimited/mit-onsite-grid-css

Add css changes from MIT's local media files