There was a first problem in the tss file as the reference to the stylesheet was missing. I fixed that using a drag and drop of the stylesheet and applying the result.
The styles are updated before the LateUpdate and before the UI is rendered. In the case submitted, style value were read before the first LateUpdate. By modifying his code, we can see that the value read during OnEnable, Start, Update are all invalid as the styles have not been calculated yet, and then they start being what the user expected.
Please note that if you set the stylesheet in the theme, it will be at the root of the panel directly, but if the stylesheet is referenced by the uxml, it will be applied on the corresponding template container (directly on the UIDocument.rootVisualElement). You will need to adjust your code accordingly. The UI Debugger is a good tool to verify that the styles are applied on the expected elements.