actually listen to react-hooks/exhaustive-deps warning

hooks/use-theme.ts

@@ -2,4 +2,12 @@ import { useContext } from "react";
 import { ThemeContext } from "../contexts/ThemeContext";
 
 // convenience hook to get access to ThemeContext's state/functions from pages/components/etc.
-export const useTheme = () => useContext(ThemeContext);
+export const useTheme = () => {
+  const context = useContext(ThemeContext);
+
+  if (!context) {
+    throw new Error("useTheme must be used inside of a ThemeProvider.");
+  }
+
+  return context;
+};