How to override gwt styles in uibinder?

I want to override button gwt styles directly in uibinder. But why does the following not work?

<g:ToggleButton styleName="{style.toggle}" />

<ui:style>
.toggle-down, .toggle-down-hovering {
  background: red !important;
  background-color: red!important;
}
</ui:style>

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