How to add a css file to the admin theme from a front subtheme?
There are a few options to accomplish this.


If your admin theme has a html.tpl.php you could manually add it in the head using &lt;link type="text/css" rel="stylesheet" href="linktoyourcssfile"&gt;


Although, know that, when you update your admin theme, the html.tpl.php file probably will be overwritten so you will need to add it every time again.




Another solution is to install the Add To Head module and add the above line to the head without having the trouble it will be overwritten the next time you update the theme.


It's up to you to see which solution fits you best.