work

#gridviewDiv {
    width: auto;
    height: 400px;
    overflow: scroll;
}

    #gridviewDiv::-webkit-scrollbar {
        width: 1em;
    }

    #gridviewDiv::-webkit-scrollbar-track {
        background-color: #393939
    }

    #gridviewDiv::-webkit-scrollbar-thumb {
        background-color: #c2c2c2;
        border: 0.2em solid #393939;
    }

<asp:TreeView ID="TreeView1" runat="server">
        <RootNodeStyle ImageUrl="/_layouts/images/SharePointProject1/folder_icon.png" HorizontalPadding="5px" />
        <LeafNodeStyle ImageUrl="/_layouts/images/SharePointProject1/file_icon.png" HorizontalPadding="5px" />
        <Nodes>
            <asp:TreeNode Text="parent1" Value="parent1" Expanded="False">
                <asp:TreeNode Text="child1" Value="child1"></asp:TreeNode>
                <asp:TreeNode Text="child2" Value="child2"></asp:TreeNode>
            </asp:TreeNode>
            <asp:TreeNode Text="parent2" Value="parent2">
                <asp:TreeNode Text="child1" Value="child1"></asp:TreeNode>
                <asp:TreeNode Text="child2" Value="child2"></asp:TreeNode>
            </asp:TreeNode>
        </Nodes>
    </asp:TreeView>

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