With the fix applied to Bug 222654 (https://bugzilla.mozilla.org/show_bug.cgi?id=222654), themes can truly be cross-platform.

Place both nativescrollbars.css and xulscrollbars.css in the global directory of your theme. Mac OS versions look for nativescrollbars.css, and all other versions look for xulscrollbars.css to find out how to draw the scrollbars.

For compatibility with versions prior to August 14, 2005 I have included the scrollbars.zip file. The scrollbars.zip file contains two versions of the scrollbars.css file. One version is for Mac OS and the other version is for all other operating systems. Firefox 1.0.x, Thunderbird 1.0.x, and Mozilla 1.7.x do not support the new scrollbars files, so separate versions of the theme must be maintained for compatibility on all operating systems. See the readme.txt file inside the scrollbars_1.0.x_.zip for instructions on deployment.