CSS Bootstrap buttons

I have a problem with stylize buttons in bootstrap. ``` <div class="btn-group"> <button class="btn btn-midd">Link</button> <button class="btn btn-midd dropdown-toggle"> <span class="icon-download-alt icon-gray"></span> </button> </div> ``` I want to change "icon-gray" to "icon-blue". Of course i have different image (and class which change backround-image). Icon-blue should be change on hover. Thanks for help. CSS: ``` .icon-blue {background-image: url("../img/glyphicons-halflings-blue.png");} .icon-download-alt {background-position: -408px -96px;} ``` Here an example :