The colours used as the background for days with special status in Datepicker (and also MonthView-in-page variant) are currently not accessible.
It's hard-coded to have transparency so will never be accessible even when darker colours are chosen.
Suggest having an outlined version as a short-term workaround until better solutions can be designed. The outline will likely need to be thick enough as well (3 or 4 px?).
Please see attached image for reference.
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The IDS product team reviews enhancement suggestions on a regular basis and implements enhancements based on assessed value. If more information is required, an IDS Product Management team member will contact you for clarification.