<aura:component implements="force:appHostable">
    <lightning:card class="test" iconName="custom:custom62" title="Test Card">
        <aura:set attribute="actions">
            <lightning:button label="Button"/>
        </aura:set>
    </lightning:card>
</aura:component>

.THIS.test {
    position: fixed;
    width: 100%;
}