What is the main theme of “the fox and the goat”?

Well to find the theme yourself you would want to look to see what the author was asking the reader or what message the story is telling the reader.

But, if you're looking for an answer exclusively, I believe the theme is along the lines of selfishness and the true nature of the human heart. Another interpretation of the theme would be to Look Before You Leap, or not be too trusting of others.

the passage can be best described as: "the british legislature extended an unwarranted jurisdiction".

we can say that because the text clearly states how the people of the colonies warned the british about their excessive desire to rule over them, without taking into consideration the difficulties their brothers had experienced in america: they struggled to establish the colonies, to adapt to new types of weather, fight with the natives, etc., and how they now see (or then saw) the only way out was to cut all ties with the british, since they ignored the people's petitions.