I posted a public example of a very simple recommended approach to centering objects vertically and horizontally through a simple sub class that can be appended to any class. I use this approach because it is generic / abstracted and can be used over and over throughout a site on any basic block element.
You have to declare a body height = 100% if you want anything to center in the browser window. or to make an object fluid to occupy a percentage of the browser height dynamically on resize. Otherwise, if you are only concerned about applying fluid width this can be applied to any object that has a fixed pixel height.
I built every element using fluid percentages for width and height. Play with them to see the results. This sounds like the missing behavior you are after.
TIP: inset the cursor in any value setter number field in webflow. Hold shift on your keyboard and use the up and down arrow keys on your keyboard to change values at 10x. Without shift it increments at 1x. Try it when adjusting percentages. It gives me instant feedback and will help you to see the behavior.
Make a new page within this example document drag a div to the stage. Set it's class to the predefined .center-all class I have created. Look at the element properties highlighted in blue. These are the settings you need to replicate to build your own.
If any class is created in abstract (no other properties for any other characteristic) it can be appended / added / tacked on to anything and invoke its behavior added to the base class. In this case it centers anything you tag it with.
Hang in there @eze81/ Learning is often a chore for me. But it is worth it to learn the right way - it is powerful and builds momentum with each step you master and really understand.
I posted the example, because you can tweak, change and break stuff to see what attributes affect the outcome. Breaking stuff is how I learn and how I know something inside out. If videos are tough for you (crazy difficult for me) maybe this approach will help.