A thesis is "a proposition for consideration, especially one to be discussed and proved or disproved." In other words, a thesis is an idea that is presented for discussion, with facts laid out to support or refute the validity of the idea.
The Declaration of Independence was written to present to the world the position that the British colonies on the North American continent were justified in declaring themselves free from the control of Great Britain.
When...it becomes necessary for one people to dissolve the political bands which have connected them with another and to assume among the powers of the earth (a) separate and equal station..., a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.
This was the purpose for the writing of the Declaration. The body of the document goes on to explain the abuses that the colonies have suffered and the efforts they have exerted to bring about a change in attitude toward them. Having failed in these efforts, the document lists "repeated injuries and usurpations" it charges "the present King of Great Britain" has commited against the colonies.
As a result of the facts presented, the colonies declare themselves justified in declaring themselves "free and independent states."