You could do this by adding a separate element to the page with your loading text and `position: fixed` CSS, and then removing that element once the game is loaded.
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I'll see if I can manage this somehow. I was able to style the progress bar itself a bit which I'm already more happy with. For me it's better recognizable as a progress bar when the loading background isn't transparent. But I'm not sure how I would remove stuff once the game is loaded. Maybe based on the loading progress percentage as a condition? Haha, guess I'll have to play around a bit.