I'm not sure why you say "in theory" - it was a well-respected plank of the colonial makeup, and black voters could exercise the franchise on exactly the same terms as white voters.
That's why at confederation they send a delegation to argue for full suffrage for peoples of all races in the new Union of South Africa - and, of course, fail.
1908 delegation from Cape Colony to the conference
Picture included mainly to make the point that Cape Colony sent a thoroughly mixed delegation to a conference in which at least two of the other states with which they would be debating had left the area of South Africa in large part to ensure they would be permitted to keep slaves.
It's true that this may not indicate
full racial equality, depending on how you define "full", but since the alternatives to "Black people may vote on the same terms as Whites" are:
1) All may vote
2) Black people may vote under less restrictive terms than whites
3) Black people may vote under more restrictive terms than whites
4) Black people may not vote
Only (1) is more equal in racial terms.
(2) and (3) are not equal - they are, in fact, discriminatory. That in one case it's positive discrimination does not change that.
(4) is of course not equal at all.