Refer below article and apply css to the button.

Get selected row in GridView on Button Click in ASP.Net

HTML

<style type="text/css">
    .Button {
        background-color: #f44336;
        color: white;
        padding: 14px 25px;
        text-align: center;
        text-decoration: none;
        display: inline-block;
    }
</style>
<asp:GridView ID="GridView1" runat="server" HeaderStyle-BackColor="#3AC0F2" HeaderStyle-ForeColor="White"
    AutoGenerateColumns="false" OnSelectedIndexChanged="OnSelectedIndexChanged">
    <Columns>
        <asp:BoundField DataField="Name" HeaderText="Name" ItemStyle-Width="150" />
        <asp:TemplateField HeaderText="Country" ItemStyle-Width="150">
            <ItemTemplate>
                <asp:Label ID="lblCountry" runat="server" Text='<%# Eval("Country") %>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:ButtonField Text="Select" CommandName="Select" ControlStyle-ForeColor="Red" HeaderText="Action" ItemStyle-Width="150" ItemStyle-CssClass="Button" />
    </Columns>
</asp:GridView>