add visual background guide for long rows

[vijayabharathib]

Hi David, thanks for your work on this site. I was using the site late at night, my eyes were tired and I kept clicking at the wrong search lens under "things you don't know". It was a little hard to trace to the lens from the topic on wide screen. I thought a small visual cue will help me click on the right one.

Issue 25 was helpful discussion, so here it is, smallest possible css to get this across. As the diff on commit would show, have used DEEP_ORANGE_100 to keep it aligned to the theme.

Just two classes' hover state added, about 10 lines of css, shouldn't add too much weight. App interactive didn't change much, though I could get 340 ms on production version of the local server and I see it aligns with actual production website on my pc (while on my dev server, app interactive was over 400 ms).

Feel free to let me know if you'd need anything changed. I know you may have lot of other things to consider before this can be merged, I'll be glad to help. Here are the results, that light orange bar under the cursor is all I thought would do. Hope it helps many.