I'd really recommend that you position the Button Bar using the available Button Bar configuration options (buttonBarPosition and buttonBarHAlign).
If you try to reposition the Button Bar manually using CSS, Juicebox will not know of this modification and will expect the Button Bar to be in a different position. This might have unforeseen knock-on effects, for example if the browser window is narrowed and the Button Bar needs to wrap onto multiple lines. (This might not apply to your own gallery with only one button on the Button Bar but it could be a concern for anyone else reading this thread with a larger Button Bar.)

Having said that, you are certainly free to try repositioning the Button Bar using CSS (but please be aware that such modifications are not officially supported).
You should be able to find out which classes and/or ids you might need to apply your custom CSS rules to (and what rules are currently being applied to specific elements) using your browser's developer tools (F12).

Try something like the following:

.jb-classifier-link-wrapper.jb-classifier-detail-area.jb-classifier-layer {
    margin-top: -10px !important;
    margin-right: -24px !important;
}