[FIX] hr_holidays: adapt stress days color to dark mode

The colors of the stress days in the calendar were not adapted for the
dark mode, they were too bright and thus unreadable.

task-3216055

closes odoo/odoo#114796

X-original-commit: 0f60e0ac2c24998c2f792a55ae282d62df6b5f42
Signed-off-by: Kevin Baptiste <kba@odoo.com>

addons/hr_holidays/static/src/views/calendar/calendar_renderer.scss

@@ -5,7 +5,8 @@
         .o_calendar_widget {
             @for $size from 1 through length($o-colors) {
                 .hr_stress_day_#{$size - 1}:not(.fc-disabled-day) {
-                    background-color: lighten(nth($o-colors, $size), 25%) !important;
+                    background: nth($o-colors, $size) !important;
+                    opacity: 0.4;
                 }
             }
         }