Add option for monochrome mode (#2464)

src/app/state/settings.ts

@@ -14,6 +14,7 @@ export interface Settings {
   useSystemTheme: boolean;
   lightThemeId?: string;
   darkThemeId?: string;
+  monochromeMode?: boolean;
   isMarkdown: boolean;
   editorToolbar: boolean;
   twitterEmoji: boolean;
@@ -47,6 +48,7 @@ const defaultSettings: Settings = {
   useSystemTheme: true,
   lightThemeId: undefined,
   darkThemeId: undefined,
+  monochromeMode: false,
   isMarkdown: true,
   editorToolbar: false,
   twitterEmoji: false,