diff --git a/quartz/util/theme.ts b/quartz/util/theme.ts
index bd0da5fb0..49cc9cce8 100644
--- a/quartz/util/theme.ts
+++ b/quartz/util/theme.ts
@@ -9,6 +9,11 @@ export interface ColorScheme {
   highlight: string
 }
 
+interface Colors {
+  lightMode: ColorScheme
+  darkMode: ColorScheme
+}
+
 export interface Theme {
   typography: {
     header: string
@@ -16,12 +21,11 @@ export interface Theme {
     code: string
   }
   cdnCaching: boolean
-  colors: {
-    lightMode: ColorScheme
-    darkMode: ColorScheme
-  }
+  colors: Colors
 }
 
+export type ThemeKey = keyof Colors
+
 const DEFAULT_SANS_SERIF =
   '-apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif'
 const DEFAULT_MONO = "ui-monospace, SFMono-Regular, SF Mono, Menlo, monospace"