Some serious errors are output as "errorboxes" with code similar to:

$OUTPUT->box(get_string('javascriptdisabledlocks', 'wiki'), 'errorbox')

or

$OUTPUT->container(get_string('err_noactivities', 'completion'), 'errorbox errorboxcontent');

or

echo $OUTPUT->box_start('generalbox errorboxcontent boxaligncenter boxwidthnormal');

I'm not sure what distinction (if any) there is between "box" and "container" and "errorbox", "erroboxcontent inside errorbox" and "errorboxcontent inside generalbox". Generally these all display as italic white writing on deep red background due to styles attached to errorbox, errorbox content.

It seems these could be replaced with calls like:

echo $OUTPUT->notification('External enrolment table not specified.', 'notifyproblem');

Which are used for similar kinds of errors, but (at least in the Standard theme) have a distinct appearance (red text on transparent background).

It would be good to standardize so that similar kinds of errors have similar PHP code as well as HTML, CSS and therefore appearance.

(Note I've got another bug about changing the $OUTPUT->notifications to a stricter API here: MDL-41608)