Is the following principle from Qur’an or Sunnah: “All acts of dunya are allowable unless prohibited.”

I am not familiar with this principle. However, there is another principle close in meaning to it which says: “All things are permissible (halaal) unless prohibited.” This is linked to things which the shariah has not prohibited or has kept quiet about and it is proven there is no harm in them. [1]

Reference:

[1] Dr Muslim bin Muhammad, Al-mumti’i fi Al-Qawaaid Al-Fiqhiyya, 141 - 145