Thales theorem is a special case of the inscribed angle theorem which states that an angle inscribed in a circle is half of the central angle that subtends the same arc on the circle. Therefore, the angle does not change as its apex is moved to different positions on the circle.
If angle inscribed in a circle and the central angle both subtend the semicircle then the central angle equals 180° and the angle inscribed equals half the central angle or 90°.