This was changed to get a good text/background contrast on the
alert-dark (Exercise).
By doing so, the .text-dark class generated by BS from the $dark
theme-color was gray instead of the intended near-black color.
By changing the variable, the contrast issue with the alert was back,
to solve this we directly override the alert's background color instead.
Part-of: odoo/documentation#3315
32c9e59ae6