Access style values in script

Whenever I try to access the styles which I set in the inspector via script I am always getting the default values.

I made a new clean project and this time I am also using my own Theme file. I added an image and gave it width 1397px, but when I access the width in my script, then its 0, which is the default value.

Here is the code and a reference image, there is literally nothing more there:

    using System.Collections;
    using System.Collections.Generic;
    using UnityEngine;
    using UnityEngine.UIElements;
    
    public class Arena_UI_Controller : MonoBehaviour {
    
        // Start is called before the first frame update
        void Start() {
            var root = GetComponent<UIDocument>().rootVisualElement;
    
            VisualElement arena = root.Q<VisualElement>("Arena");
            Debug.Log(arena.style.width.value); // Should be 1397px
        }
    
        // Update is called once per frame
        void Update() {
        
        }
    }

Also, what is the best practice with the Theme Style Sheets? Per default it is always the UnityDefault, but should I create my own for every seperate UI Document?

Here is also a a screenshot of the panel settings:

arena.style is only the inline style, which doesn’t account for all the stylesheets and other things that can affect the final value. You might want to try arena.resolvedStyle instead. Also keep in mind that there can be a 1-frame delay between the moment a style is set and the moment it’s properly resolved.