Air desktop application CSS file and image url

Question

I've got a StyleableTextField that displays very basic HTML. This css file is calling on main class like this :

<fx:Style source="assets/skin/1styles.css"/>

My project directory is like this

MainLevel

  ____ src
       mainApp.mxml
       ____ assets
                    ____ skin
                                  1styles.css
                                  ____ assets

For example with this code, an error appear about wrong path

.nestedTabButton {
    skin: Embed("./assets/blank.png");
    font-weight: normal;
    text-roll-over-color: #CCCCCC;
    text-roll-over-color: #CCCCCC;
}

So could you help me to find the good path ?

Best regards