[IMP] odoo-theme: Revert $dark color variable to initial $gray-900

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.

X-original-commit: 32c9e59ae6
Part-of: odoo/documentation#3320

extensions/odoo_theme/static/scss/bootstrap_overridden.scss

@@ -26,7 +26,7 @@ $teal:                                      #017e84;
 $primary:                                   $teal;
 $secondary:                                 $purple;
 $light:                                     $gray-100;
-$dark:                                      $gray-600;
+$dark:                                      $gray-900;
 $success:                                   $green;
 $info:                                      $blue;
 $warning:                                   $orange;

extensions/odoo_theme/static/style.scss

@@ -994,6 +994,8 @@ header {
             &.alert-dark {
                 --o-alert-icon: '#{$i-exercise}';
                 --o-alert-code-border-color: #{tint-color($dark, 50%)};
+
+                background: $gray-300;
             }
         }
         .graphviz {