I have been trying out Google Messages and have the Beta version. This is how it is working so far for me. I have the ability to enable Chat/RCS in settings. However, those features do not work for me yet across the divide with iPhone users. I have read that they should work if I message other Android Google Messages Beta users, but I don't yet have any message with other Beta users yet that I am aware of. So even though Chat features are enabled, I have not seen anything of them yet in any of my messages.
There are no additional skinning, theme or color customization options available in settings on my Beta version, other than the Light or Dark Themes. I really do wish Google Messages had more visual customizations. I am OK with the green and gray bubbles in messages but it would be nice to be able to change those. What I really don't like about the visual interface is the Home Screen. On either the light or dark theme, there is really no differentiation between the message threads in the list. It is just a flat panel visually.
Also, I don't like the way that the Home Screen keeps dropping down so that the top 1/2 of the page is a big wasted space with a "Messages" header and then the number of unread messages. You can drag the messages up so you are using the entire real estate of the home screen, but it wants to keep jumping back to the top, with that 1/2 page of empty space. To be fair, I think the Samsung Messages app home page does the same thing.
I also wish there was a supported stand alone Windows app. For example, like the one MySMS uses. You can have a tab open in a browser with your messages, but that is all.