Fix colors on the proctor login page for dark mode. (Bootstrap alert aproach)

[drgrice1]

The colors for this were inline style. Colors cannot be inline anymore since the server cannot detect if the user will have the browser in dark mode or not. So this switches to using Bootstrap alerts instead.

This is an alternate approach to #2967.

One minor problem with this approach is that in the math4-yellow theme the difference between an alert-primary and an alert-warning is somewhat subtle. Here is a screenshot demonstrating the two together with this pull request:

The first alert in that image is an alert-warning and the second is an alert-primary.

[somiaj]

Seems that alerts looking similar might also be an issue on the ProblemSet page or other pages with multiple alerts.

My thoughts (Since I requested this) is this approach is preferable, let's stick with the bootstrap colors for consistency vs have some pages use their own.

[drgrice1]

This isn't really a problem elsewhere. Just because we don't use alert-primary anywhere else. This could be switched to using alert-secondary or alert-info, both of which are similar in the math4-yellow theme, but quite different from alert-warning. The math4-yellow theme colors could also be adjusted. At this point only the primary and info colors are changed from the Bootstrap defaults.

[Alex-Jordan]

I would go with this one. But since @drgrice1 uses the yellow theme, I defer to you if the subtleness is too much of a concern. I'll approve both and let you merge whichever.

[drgrice1]

I think I also prefer this approach. The subtle difference for the math4-yellow theme is enough for me. Particularly for a page that isn't used much.