There is a good guide to powerpoint accessibility generally here: http://webaim.org/techniques/powerpoint/
It goes to adding alt tags, reading order and (briefly) to contrast in colour etc.
As does this overview: https://www.slidegenius.com/blog/making-your-powerpoint-accessible-for-the-visually-impaired
Frankly all our materials should take these things into account... Usability by design goes to not just the technology but how we communicate about the technology.
regards
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Narelle Clark
narellec@xxxxxxxxx