I have green buttons under Safari, Chrome, FireFox, Opera on Mac and internet explorer, Edge, FireFox, Opera on Windows that turn grey under Android in both the native browser and Chrome. I tried to apply a gradient but no luck.
Is that a know issue ?
This was covered 2 years ago here:
https://forum.xojo.com/5801-webbutton-web-styles-on-android/0
Reason:
Because we cannot apply styles without DOM hacks to child DIVs, buttons and such controls have the underlying styles applied to a DIV within the control DIV, by Xojo. The Xojo webstyler only applies styles to the outter DIV of a control ID. Android, which uses more refined CSS (everyone else is still catching up), requires styles to applied to DIV’s by classname specification, so that child DIVs do not inherit the parent styles (this has been an issue, which is why the refinement will eventually spread across the board). Because of this, background styles cannot be applied to controls such as WebButtons for android devices, on Xojo currently; the DOM structure for Android is very specific. We can use however, a WebLabel and apply a button style to turn the label into a WebButton, whos background style is accessible, and proper for all devices. In place of the Action event, simply use a WebLabel’s MouseDown event.
Numerous bug reports have already been filed for this.
OK. Replaced by labels as advised in the other thread, now they show up fine in Android 
Thanks.