diff --git a/emmental/__init__.py b/emmental/__init__.py
index d69ed56..3f552dc 100644
--- a/emmental/__init__.py
+++ b/emmental/__init__.py
@@ -29,6 +29,7 @@ class Application(Adw.Application):
     def do_startup(self) -> None:
         """Handle the Adw.Application::startup signal."""
         Adw.Application.do_startup(self)
+        gsetup.add_style()
 
     def do_activate(self) -> None:
         """Handle the Adw.Application::activate signal."""
diff --git a/emmental/emmental.css b/emmental/emmental.css
new file mode 100644
index 0000000..65bce02
--- /dev/null
+++ b/emmental/emmental.css
@@ -0,0 +1 @@
+/* Copyright 2022 (c) Anna Schumaker. */
diff --git a/emmental/gsetup.py b/emmental/gsetup.py
index b102342..7f54579 100644
--- a/emmental/gsetup.py
+++ b/emmental/gsetup.py
@@ -1,13 +1,29 @@
 # Copyright 2022 (c) Anna Schumaker.
-"""Set up GObject Introspection."""
+"""Set up GObject Introspection and custom styling."""
+import pathlib
 import sys
 import gi
 
+gi.require_version("Gdk", "4.0")
 gi.require_version("Gtk", "4.0")
 gi.require_version("Adw", "1")
 
+gi.importlib.import_module("gi.repository.Gtk")
+
 DEBUG_STR = "-debug" if __debug__ else ""
-APPLICATION_ID = f"com.nowheycreamery.emmental{DEBUG_STR}"
+APPLICATION_ID = f"com.nowheycreamery.emmental{'-debug' if __debug__ else ''}"
+
+CSS_FILE = pathlib.Path(__file__).parent / "emmental.css"
+CSS_PRIORITY = gi.repository.Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION
+CSS_PROVIDER = gi.repository.Gtk.CssProvider()
+CSS_PROVIDER.load_from_path(str(CSS_FILE))
+
+
+def add_style():
+    """Add our stylesheet to the default display."""
+    style = gi.repository.Gtk.StyleContext
+    style.add_provider_for_display(gi.repository.Gdk.Display.get_default(),
+                                   CSS_PROVIDER, CSS_PRIORITY)
 
 
 def __print_version(subsystem, major, minor, micro):
diff --git a/tests/test_gsetup.py b/tests/test_gsetup.py
index 1501ac2..e0f0ec4 100644
--- a/tests/test_gsetup.py
+++ b/tests/test_gsetup.py
@@ -1,6 +1,7 @@
 # Copyright 2022 (c) Anna Schumaker.
 """Test that the gi.repository has been configured correctly."""
 import unittest
+import pathlib
 import emmental
 import gi
 
@@ -28,3 +29,12 @@ class TestGSetup(unittest.TestCase):
         self.assertEqual(emmental.gsetup.DEBUG_STR, "-debug")
         self.assertEqual(emmental.gsetup.APPLICATION_ID,
                          "com.nowheycreamery.emmental-debug")
+
+    def test_css(self):
+        """Check that our custom stylesheet is configured."""
+        path = pathlib.Path(emmental.__file__).parent / "emmental.css"
+        self.assertEqual(emmental.gsetup.CSS_FILE, path)
+        self.assertEqual(emmental.gsetup.CSS_PRIORITY,
+                         gi.repository.Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION)
+        self.assertIsInstance(emmental.gsetup.CSS_PROVIDER,
+                              gi.repository.Gtk.CssProvider)