Your project is call myProject then you should have a css file called myProject.css and this css file should be added to your projects html file, etc..

Put this in that css file:

    .transparent .x-tab-panel-body-top {
        background-color: transparent !important;
    }

then add a the style name to a TabPanel.

    TabPanel tabs = new TabPanel();
    tabs.addStyleName("transparent");

Inspiration: https://extjs.net/forum/showthread.php?p=333588