UI Toolkit: Is it possible to modify USS custom properties (variables) with C#?

Hello, I'm trying to create a crosshair with the new UI toolkit package but I cannot find a way to modify my crosshair variable var(--gap) from within C#.

I can read the value just fine (prints out 550%), but when I try to change it in game nothing happens. Confirmed by the UI debugger window.

Crosshair.cs

 private VisualElement _root;
 private CustomStyleProperty<string> _gap = new CustomStyleProperty<string>("--gap");
 
 private void Start()
 {
     var uiDocument = GameObject.Find("UIDocument");
     _root = uiDocument.GetComponent<UIDocument>().rootVisualElement;
     _root.RegisterCallback<CustomStyleResolvedEvent>(OnCustomStyleResolved);
 }
 
 private void OnCustomStyleResolved(CustomStyleResolvedEvent  e)
 {
     if (!e.customStyle.TryGetValue(_gap, out var gap)) return;
         gap = "0%";
 }
 ...

Crosshair.uss

 .crosshair {
     background-color: rgb(0, 255, 17);
     position: absolute;
     --length: 400%;
     --thickness: 40%;
     --minThickness: 1px;
     --minLength: 10px;
     --gap: 550%;
 }
 ...