请教 Safari 通过外挂 .css 指定网页字体的问题

不行，因为你这个css本来就无效..
html, body, div, span, applet, object, iframe, h1, h2, h3,
h4, h5, h6, p, blockquote, pre, a, abbr, acronym,
address, big, cite, code, del, dfn, em, font, img, ins,
kbd, q, s, samp, small, strike, strong, sub, sup, tt,
var, dl, dt, dd, ol, ul, li, fieldset, form, label, legend,
table, caption, tbody, tfoot, thead, tr, th, td{font-family: "Helvetica Neue", "Hiragino Sans GB" !important;}

LZ 可以装一个 stylish 扩展。
http://sobolev.us/stylish/