Replace loading gif

I am trying to replace the default loading icon.

I have put new gifs in /theme/[theme name\]/core/img and added the following lines to /theme/[theme name]/css/server.css

img, object, video, button, textarea, input, select {
        .icon-loading {
                background-image: url('../img/loading.gif');
        .icon-loading-dark {
                background-image: url('../img/loading-dark.gif');
        .icon-loading-small {
                background-image: url('../img/loading-small.gif');
        .icon-loading-small-dark {
                background-image: url('../img/loading-small-dark.gif');

Unfortunately the Developer Tool in Chrome doesn’t really show me which loading.gif is loaded. Has anybody done this successfully?