Adding dashicon fonts to the admin of pre 3.8 installs

This happens sometimes when the font is sent with a wrong MIME type. application/x-font-woff for example, is wrong.
Try to add proper MIME types to your server configuration. In Apache, your can do that in a .htaccess:

AddType image/svg+xml                 .svg
AddType application/x-font-ttf        .ttf
AddType application/x-font-opentype   .otf
AddType application/vnd.ms-fontobject .eot
AddType application/font-woff         .woff

You should add instructions for your users to the theme, because a theme should not touch server configuration files.