How to correctly change the style of ProgressBar

So I got a little progress bar set up in my game, which represents the players stamina. It works great and now I want to change the style of it, but I see that it wont be such an easy task. After some digging I found that there are websites in where you can customize the style and all, but now the question - How do I load all that into the progress bar so that it would change its looks ?

There are a few tutorials:

construct.net/en/tutorials/how-to-css-style-buttons-sliderbar-etc-687

construct.net/en/tutorials/css-your-buttons-and-textboxes-818

But I really recommend making the progress bar with a sprite, it's so much easier and the result will be better.

In the end, I did it as you wrote. Thanks again, much easier and simpler. Though i used a different formula which suits the game quite well - Player.stamina * 2