I know that the orgin of the screen space is at the bottom left of the screen but because of some aspects of the game I’m making I need to modify it to like (0; 100)(In accordance with the origin)

Create your own static class to wrap the functionalities about screenspace you need. It’s a bit vague for an answer, I (or someone else) could explain further if you wish.

well I searched and find out there is a math fomula to calculate lines in 2d space, you have to implement it manually.

The equation represent a line is : ax + by + c = 0, ex : 3x + 4y - 5 =0;

so in my case line y = 100 is actually 0x + 1y -100 = 0 and the other line is (b-100)x + ay + c =0

(let (a, b) be mouse position on screen) , fomula to caculate angle between line : (a1*a2 + b1*b2)/(sqrt(a1^2 + b1^2) + sqrt(a2^2 + b2^2))

if line1 : a1x + b1y + c1 = 0

line2 : a2x + b2y + c2 = 0

we dont really care about "c"s in this case