A button is a filled rectangle with text in it. In most screens, this is great, but sometimes a button shouldn't look like a button. It should look like clickable text. This is a design decission, but check for yourself:

It's easy to style a button like text. Simply set following style:

Style.addStyleNames(button, "f_text");

If the text color should be the same as the color of a label, use:

Style.addStyleNames(button, "f_text_label");

and the text color of the button will be the same as a standard label/text and it will be changed in dark mode automatically.