Are all templates accepted?

No. We have a high bar for our templates and each goes through at least one round of full code review with the MUI team and sometimes multiple rounds. We're aiming to do this as efficiently as possible but the end user's experience and having a quality of templates in the marketplace are of primary importance.

See What does a great template look like? for what we are looking for at a high level.

We've got a longer, more technical document outlining the whole process for review in the following article so take a look at it when you've decided you can hit the points above and follow the directions to submit your template: Review checklist for new items.