fix(lib): correctly retrieve background color (#414)

Closes #409

manim_slides/slide/manim.py

@@ -23,7 +23,7 @@ class Slide(BaseSlide, Scene):  # type: ignore[misc]
 
     @property
     def _background_color(self) -> str:
-        color = config["background_color"]
+        color = self.camera.background_color
         if hex_color := getattr(color, "hex", None):
             return hex_color  # type: ignore
         else:  # manim>=0.18, see https://github.com/ManimCommunity/manim/pull/3020

tests/test_slide.py

@@ -6,6 +6,7 @@ import numpy as np
 import pytest
 from click.testing import CliRunner
 from manim import (
+    BLACK,
     BLUE,
     DOWN,
     LEFT,
@@ -108,6 +109,13 @@ class TestSlide:
             assert len(self._canvas) == 0
             assert self._wait_time_between_slides == 0.0
 
+    @assert_constructs
+    class TestBackgroundColor(Slide):
+        def construct(self) -> None:
+            assert self._background_color == BLACK.to_hex()  # DEFAULT
+            self.camera.background_color = BLUE
+            assert self._background_color == BLUE.to_hex()
+
     @assert_renders
     class TestMultipleAnimationsInLastSlide(Slide):
         """Check against solution for issue #161."""