There are three theocracies on Midrea, the enormous Lawbringer Theocracy, Vale of the Just and the Queen Domains but only the last two which are small are ruled entirely by spell casters.
There are two main reasons for this
#1 Clerics and such are few in number
#2 The Gods have other work for them
The Lawbringer Theocracy has a complex bureaucracy something akin to a cross between the Vatican and the ancient Chinese system but the bulk of it is controlled by educated Experts and a smattering of other classes. While Clerics do occupy higher ranks, there simply are not enough of them to run a nation of millions, so the leadership is a bit more pyramidal with Clerics at the top, Oracles advising and Paladins and Inquisitors in the field. The day to day work falls to the regular folks.
The Vale of the Just is a tiny city state of maybe 20k whose ruling council are all Paladins and Clerics. because of its tiny size and the location of a holy site, the areas produces enough spell casters to do the day to day work. This is a rare event though and it wouldn't scale.
The Queens Domain is akin to a Dianic Wiccan Theocracy . Its run by a Grand Council of Witches divided into 4 groups, The Elders, The Matrons, the Maidens and the advisory mens group, the Consorts .
Like the Vale its a small nation and blessed with a number of holy sites giving it enough ambient magic to allow a well above average percent of the population to have magical abilities . Since Wicca can be taught, it is the case that all the Women have some levels in Witch as a class.
They aren't all good at it but with concentrated effort its been possible to get 10% of the female population trained and initiated into the mysteries. This leaves enough for a ruling class of sorts. Note that none of the men have any caster levels (or at least if they do they are well hidden) and they have no political power to speak of although they can advise.
The Return of Planescape? - Curious. There is a brief note on this 'countdown' at Enworld.
13 hours ago