Redmine

Thank you, this looks great, and moving the bars from images to CSS is a real improvement for themes.

One suggestion before it lands: the selectors now refer to Open Color directly, for example .gantt-task-done { background-color: var(--oc-green-6) }. The value is no longer hard coded, but the choice of colour still is, because it moves into the name. A theme that wants a different colour for "done" has to override every selector that uses --oc-green-6 for that purpose, and it cannot change --oc-green-6 itself without changing everything else that is green.

Could the Gantt colours be defined by their role instead, once, and the selectors use the roles?

:root {
  --gantt-task-done:      var(--oc-green-6);
  --gantt-task-late:      var(--oc-red-4);
  --gantt-task-todo:      var(--oc-gray-5);
  --gantt-task-parent:    var(--oc-gray-6);
  --gantt-task-marker:    var(--oc-indigo-8);
  --gantt-version-marker: var(--oc-gray-9);
  --gantt-today-line:     var(--oc-red-7);
}

.gantt-task-done { background-color: var(--gantt-task-done); border-color: var(--gantt-task-done); }

Open Color stays the palette and the defaults are exactly the ones in the patch, so nothing changes visually. What it adds:

• A theme, or a dark mode, changes what "done" or "late" looks like in one place, without touching the selectors or the palette.
• The meaning is in the name, so someone reading the CSS sees why a bar has its colour.
• The list of roles documents the Gantt colours in one place.

The same idea could apply to the PDF and PNG exports: one map from role to colour per output (for example a constant with :done, :late, :todo, :marker) instead of values spread over the drawing code. That would also make it explicit where screen, PDF and PNG differ on purpose.

If this sounds right, it might be worth doing the same for other areas in a follow-up to #43256, but for this patch the Gantt roles alone would already help.

But please, implement this patch and we might discuss my idea afterwards since this is a bigger question than this patch. :)