There are a couple of bugs with buttons on toolbars when using the css for 6.3 when choosing the flat style.
First, when using the webix_transparent css with icon buttons, the icon color is black, and the toolbar is blue. Seems like the color for material was carried over to flat by mistake. Looks good with material, but with flat this is very hard to see, and for color blind persons, it would be invisible. The icon should be white. See
https://snippet.webix.com/2pwhxmg2
Second, when using the webix_secondary css (or the default if no css is specified), the icon button has the correct white color, but has a border that was not there in 6.2 … perhaps this was intentional, but there really should be no border in this case. It clutters things up and distracts from the icon. See
https://snippet.webix.com/2dqiqyrl
Thanks.