Just sticking this at the end of the CSS file seems to help (obviously the 800px...

Just sticking this at the end of the CSS file seems to help (obviously the 800px could be adjusted downward if that is deemed too wide for filtering mobile devices):

  @media screen and (max-width: 800px), handheld and (max-width: 800px) {
    img[src="grayarrow.gif"], img[src="graydown.gif"] {
      width: 20px;
      height: 20px;
    }
  }

(It'd be nice if the images were unicode symbols, e.g. http://www.fileformat.info/info/unicode/char/25b2/index.htm)

And possibly putting the following in the <head>, so that the screen starts zoomed to a reasonable level.

  <meta name="viewport" content="width=device-width, initial-scale=1">