CSS Style for ui:button in Salesforce Lightning Component non rendering

Hi All,

I followed the instructions in the Lightning Components Developer Guide for applying CSS to a ui:button in my Lightning component, but the style isn't showing up.

Here's the button in my component:

<ui:button label="Box Volunteer Folder" press="{!c.navigate}" class="btn" />

Here's the CSS:

.THIS.uibutton.btn {
background-color: #2574a9;
color: #fff;
padding: 10px 20px;
}

I've also tried this with no success:

.THIS .uibutton.btn {
background-color: #2574a9;
color: #fff;
padding: 10px 20px;
}

Thanks