1 thing would be not to include useless widgets like your name or the date.
I’m assuming that the icons with no images on are just placeholders at the moment, but maybe stick to 1 colour scheme throughout your UIs. I see red, cyan and purple all mixed together, and it looks really messy.
Maybe go for a “less is more” type of layout. As for the Coins and Stage stats, you should just merge them into 1 item instead of using 2 separate text boxes. They look kind of awkward right now.
If you want to make red your main theme, I would go for a softer shade, preferably with the background of the buttons transparent to show the main background.
UI Stroke (You can make a script such that is appears when a player hovers over it, but make sure your colours match.)
Background with gradients (You can do this, just make sure not to overuse it.)
Title Bar (What do you mean by this? Is it like the game title as part of the UI above screen?)
Transparency (Sure, especially with the button’s background.)
Colour (You will need to decide that on your own. There’s no “popular” colour.)
I think the best thing to do here would be to focus on colour matching first. For example, if you are going to red, stick to using red and maybe 1 other colour. If you can sort this out first, then you can focus on making popular UI themes.