Altering a thesis after acceptance? I am told that any changes in content beyond correction of small mistakes, like obvious wrong use of a term or typographics is not allowed. This makes sense to me.

Is this a common opinion or are there institutes somewhere where no chances at all are allowed?

Of course one can ask, why the alterations after acceptance of a thesis. But let's assume perfectionist reasons and in case one wants to show it to potential employers where some mistake might be embarrassing.
When I submitted my MSc thesis, I thought my thesis was perfect (English/vocab/grammar). Now, every time I go back and read a section or chapter, I find some mistakes/things I will never use if I'm to write it now! I'm sure this will always be the case since your writing style/opinion will evolve and change. The point is to try to be better in your current work (thesis, papers etc). I wouldn't bother going back to FIX stuff but rather to see how much I have become better!