Set Theme property of the MobilePage.

Code:

    <mob:MobilePage ID="mbPage1" runat="server" Theme="B">
        <Header>
            <h1>Theming Page</h1>
        </Header>
        <Content>
            <p>Theming page presentation:</p>
            <p>
            <mob:LinkButton ID="LinkButton1" runat="server" Text="Link button with theme b" Url="#"/>
            </p>
        </Content>
        <Footer>
            <h1>Footer</h1>
        </Footer>
    </mob:MobilePage>

Result: