I modified the project I uploaded earlier to show the result after modification.
I my case, I use the webview control to display the content of a chat conversation. With this method you can style the conversation as needed and the webview will be displayed with a transparent background.
It's important to set the background of the webview to black, and the blending mode of the webview to SCREEN, then you can style the rest as wanted. The issue is that you won't be able to display black text. If you need to display black text, you will need to set the background to #FFFFFF white, then set the blending mode to MULTIPLY. More on blending modes here :
https://en.wikipedia.org/wiki/Blend_modes#Screen
Although this method isn't perfect, it fits my needs. I still think java should release an option to make the background transparent.
Thanks Erel for helping me.