Customized web publishing

Hi folks,
I have made a game that I want to publish on a website. An early and not final Swedish version of the game can be viewed here: (the option to run an English version will come later).

Now I would like to do some settings for publishing on a website. Is it possible to center the scene (or place it where ever I want) in the browser window? Is there a way to mask all content that is outside the scene? Maye have a tiled image around the scene filling up all the empty space. Currently I have placed a big black image on the right side of the scene, but that feels a bit workaround-ish. Are there other settings or actions I can use for web publishing? I’m not good at JS and CSS but I guess I could learn some basics, if there are no native settings for this in GDev.