diff --git a/shiny/ui/_theme.py b/shiny/ui/_theme.py
index 5225d8b1..d0fb6316 100644
--- a/shiny/ui/_theme.py
+++ b/shiny/ui/_theme.py
@@ -32,13 +32,6 @@ from ._utils import path_pkg_www
 T = TypeVar("T", bound="Theme")
 
 
-SassImporterReturnValue = Union["tuple[str]", "tuple[str, str]", "tuple[str, str, str]"]
-SassImporterFunction = Union[
-    Callable[[str], SassImporterReturnValue],
-    Callable[[str, str], SassImporterReturnValue],
-]
-
-
 class SassCompileArgs(TypedDict):
     output_style: NotRequired[Literal["nested", "expanded", "compact", "compressed"]]
     source_comments: NotRequired[bool]
@@ -50,7 +43,7 @@ class SassCompileArgs(TypedDict):
     precision: NotRequired[int]
     custom_functions: NotRequired[Any]  # not worth the effort, it's a complicated type
     indented: NotRequired[bool]
-    importers: NotRequired[Iterable[tuple[int, SassImporterFunction]] | None]
+    importers: NotRequired[Any]  # not worth the effort, it's a complicated type
 
 
 theme_temporary_directories: set[tempfile.TemporaryDirectory[str]] = set()
@@ -416,16 +409,23 @@ class Theme:
         check_libsass_installed()
         import sass
 
-        if compile_args is None:
-            compile_args = {"output_style": "compressed"}
+        args: SassCompileArgs = {} if compile_args is None else compile_args
+
+        if "include_paths" in args:
+            raise ValueError(
+                "The 'include_paths' argument is not allowed in 'compile_args'. "
+                "Use the 'include_paths' argument of the Theme constructor instead.",
+            )
+
+        args: SassCompileArgs = {
+            "output_style": "compressed",
+            "include_paths": self._include_paths,
+            **args,
+        }
 
-        self._css = cast(
-            str,
-            sass.compile(
-                string=self.to_sass(),
-                include_paths=self._include_paths,
-                **compile_args,  # type: ignore
-            ),
+        self._css = sass.compile(
+            string=self.to_sass(),
+            **args,
         )
 
         return self._css