Since a refactoring of the handling of the color system and the way the text
muted is handled, the text-muted isn't visible on the full-screen mode of a
course. That is becuase text-muted is based on the font color set for the body,
which in this case is quite similar to the background of left panel in
full-screen course content.

This commit fixes the issue by using `text-600` class that is suited for this
scenario (the background color is fixed here irrespective of the color-palette,
and so should be muted text-color) instead of `text-muted` for the left panel
of full-screen course content.

taskId - 2346013
closes #59390

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>