change background color in manim

set_background.py

from big_ol_pile_of_manim_imports import *

def set_background(self):
    background = Rectangle(
    width = FRAME_WIDTH,
    height = FRAME_HEIGHT,
    stroke_width = 0,
    fill_color = "#3E746F",
    fill_opacity = 1)
    self.add(background)


class background_setter(Scene):
#A few simple shapes
    def construct(self):
        #set background color
        set_background(self)
        self.wait(2)

@jaddoescad, I'm really sorry for the stupid question but I'm a newbie, how should I use this py file in a separate project, in a different Python script?

You can import the file the same way you import a library using import command. See this tutorial: https://www.pythoncentral.io/how-to-create-a-python-package/