theme types: Fix themeColors annotation.

We'll want to deprecate the 'default' name; as I explain on zulip#4009
[1], any 'default' semantics should only apply the device level, at
least on iOS, not within the app.

[1]: zulip#4009

src/styles/theme.js

@@ -2,14 +2,16 @@
 import React from 'react';
 import type { Context } from 'react';
 
+import type { ThemeName } from '../reduxTypes';
+
 export type ThemeColors = {|
   color: string,
   backgroundColor: string,
   cardColor: string,
   dividerColor: string,
 |};
 
-export const themeColors: { [string]: ThemeColors } = {
+export const themeColors: { [name: ThemeName | 'light']: ThemeColors } = {
   night: {
     color: 'hsl(210, 11%, 85%)',
     backgroundColor: 'hsl(212, 28%, 18%)',