Enhancement: allow usage of custom Pygments styles in Code (#980)

Author: Theaninova

manim/mobject/svg/code_mobject.py

@@ -206,7 +206,8 @@ def __init__(
             raise ValueError(
                 "Neither a code file nor a code string have been specified."
             )
-        self.style = self.style.lower()
+        if isinstance(self.style, str):
+            self.style = self.style.lower()
         self.gen_html_string()
         strati = self.html_string.find("background:")
         self.background_color = self.html_string[strati + 12 : strati + 19]