suppress background highlight on first line

[kevinushey]

Some Ace highlight rules will also give tokens a background highlight color. For example, Stan's highlight rules (in RStudio) give var a red background highlight. Unfortunately, these highlight rules also apply to tokens on the first line, and so we can end up with a chunk like so:

This PR alleviates that by forcing background: none !important in the CSS for panmirror.

[cscheid]

Is there any reason we'd ever want the first line to be highlighted?

[kevinushey]

Is there any reason we'd ever want the first line to be highlighted?

I don't think so, but it's plausible that we'll want to some day adjust the treatment of the chunk header in the future? That seems like an overall larger project though.