message: Fix custom theme use with new room list
  Fixes
  We were grabbing "custom-" instead of the actual theme name.
diff: @@ -80,7 +80,7 @@ export default class UserMenuButton extends React.Component<IProps, IState> {
  private isUserOnDarkTheme(): boolean {
  const theme = SettingsStore.getValue("theme");
  if (theme.startsWith("custom-")) {
  - return getCustomTheme(theme.substring(0, 7)).is_dark;
  + return getCustomTheme(theme.substring("custom-".length)).is_dark;
  }
  return theme === "dark";
  }
  