If you modify or redefine (recommended) the vertical filling of line 18 of jquery.ui.button.css, you can fix it. Here is the current block:
.ui-button-text-only .ui-button-text {
padding: 0.4em 1em;
}
Here's a potential fix:
.ui-button-text-only .ui-button-text {
padding: 0.3em 1em;
}
Webkit, , .