I can confirm this crash with the current master. This can be reproduced by attempting to drag any color over to the outliner, not just from the theme preferences.

@Paul Muns (dznwks) In case this wasn't clear, the theme settings only allow you to configure the preset colors. Assigning a color to a collection is done by right-clicking on a collection in the outliner and then picking one of the color tags.

datastack_drop_init(bContext * C, const wmEvent * event, StackDropData * drop_data) Line 715	C
datastack_drop_poll(bContext * C, wmDrag * drag, const wmEvent * event, const unsigned char * * r_tooltip) Line 859	C
dropbox_active(bContext * C, ListBase * handlers, wmDrag * drag, const wmEvent * event) Line 208	C
wm_dropbox_active(bContext * C, wmDrag * drag, const wmEvent * event) Line 238	C
wm_drop_operator_options(bContext * C, wmDrag * drag, const wmEvent * event) Line 264	C
wm_drags_check_ops(bContext * C, const wmEvent * event) Line 284	C
wm_event_do_handlers(bContext * C) Line 3365	C
WM_main(bContext * C) Line 486	C
main(int argc, const unsigned char * * UNUSED_argv_c) Line 523	C

Sorry Robert, now that I see how beautiful the menu is after RT-clicking the Collection, I feel kinda dumb. However, so many graphics programs allow this kind of drag'n'drop that it is like an atavistic action to try it. I hope it is easy to fix.

It appears that datastack_drop_poll doesn't check that drag->type matches the expected type and always interprets the pointer in drag->poin as a pointer to StackDropData. This is obviously not correct in case the drag and drop is for e.g. color. Interpreting the memory it points to as pointer and attempting to dereference it in datastack_drop_init by using drop_data->drag_tselem->type results in the segmentation fault.

Adding the following check to datastack_drop_poll should avoid this issue:

if (drag->type != WM_DRAG_ID) {
   return false;
}

I will post a diff shortly.