<TelerikToolBar>
            <ToolBarButton Class="my-custom-class" ...  />
            <ToolBarTemplateItem Class="my-custom-class">
...
            </ToolBarTemplateItem>
</TelerikToolBar>

<style>
    .second-template-item {
        border: solid;
        border-color: limegreen;
    }

    .my-group button:nth-child(1) {
        border: solid;
        border-color: red;
    }

    .my-group button:nth-child(3) {
        border: solid;
        border-color: blue;
    }
</style>

<TelerikToolBar>
    <ToolBarButtonGroup Class="my-group">
        <ToolBarButton>Bold</ToolBarButton>
        <ToolBarButton>Italic</ToolBarButton>
        <ToolBarButton>Underline</ToolBarButton>
    </ToolBarButtonGroup>
    <ToolBarSeparator />
    <ToolBarTemplateItem>
        <TelerikDropDownList Data="@Roles" @bind-Value="@SelectedRole" />
    </ToolBarTemplateItem>
    <ToolBarTemplateItem>
        <TelerikDropDownList Class="second-template-item" Data="@Roles" @bind-Value="@SelectedRole" />
    </ToolBarTemplateItem>
</TelerikToolBar>

@code {
    public string SelectedRole { get; set; }

    public List<string> Roles { get; set; } = new List<string>()
    {
         "Manager", "QA", "Developer", "Support"
    };

    protected override void OnInitialized()
    {
        SelectedRole = Roles.FirstOrDefault();
    }
}