To have a consistent look in grids toolbar, it should be possible to add an icon class to custom buttons. Not possible through .HtmlAttributes() because those gets applied to the wrapper link and kendo icons are displayed via inner span tag.

.ToolBar(toolbar =>
    {
        toolbar.Create(); // Has [+]-Icon
        toolbar.Custom().Text("My Custom Button"); // Does not have an icon
    }

Would be nice to have an option like

toolbar.Custom().Text("Reset Grid Settings").Icon("k-i-funnel-clear");

which results in 

<a href="/Tarife" class="k-button k-button-icontext">
    <span class="k-icon k-i-funnel-clear"></span>
    Reset Grid Settings
</a>