Unity 6.3 Custom Main Toolbar

MainToolbarButtons.cs

using UnityEditor;
using UnityEditor.Toolbars;
using UnityEngine;
using UnityEngine.UIElements;

public class MainToolbarButtons
{
    [MainToolbarElement("Project/Open Project Settings", defaultDockPosition = MainToolbarDockPosition.Middle)]
    public static MainToolbarElement ProjectSettingsButton()
    {
        return new MainToolbarButton
        (
            new MainToolbarContent(EditorGUIUtility.IconContent("SettingsIcon").image as Texture2D),//Content(Image)
            () => { SettingsService.OpenProjectSettings(); }//Action
        );
    }

    [MainToolbarElement("Timescale/Reset", defaultDockPosition = MainToolbarDockPosition.Middle)]
    public static MainToolbarElement ResetTimeScaleButton()
    {
        MainToolbarElementStyler.StyleElement<UnityEditor.Toolbars.EditorToolbarButton>("Timescale/Reset", element =>
        {
            element.style.paddingLeft = 0f;
            element.style.paddingRight = 0f;
            element.style.marginLeft = 0f;
            element.style.marginRight = 0f;
            element.style.minWidth = 20f;
            element.style.maxWidth = 20f;

            var image = element.Q<Image>();
            if (image == null) return;
            image.style.width = 12f;
            image.style.height = 12f;
        });

        return new MainToolbarButton(
            new MainToolbarContent(EditorGUIUtility.IconContent("Refresh").image as Texture2D, "Reset"),//Content(Image, Tooltip)
            () => { Time.timeScale = 1f; MainToolbar.Refresh("Timescale/Slider"); }//Action
        );
    }
}

MainToolbarElementStyler.cs

using UnityEditor;
using UnityEngine;
using UnityEngine.UIElements;

public static class MainToolbarElementStyler
{
    public static void StyleElement<T>(string elementName, System.Action<T> styleAction) where T : VisualElement
    {
        EditorApplication.delayCall += () =>
        {
            ApplyStyle(elementName, (element) =>
            {
                T targetElement = element is T typedElement ? typedElement : element.Query<T>().First();
                if (targetElement == null) return;
                styleAction(targetElement);
            });
        };
    }

    static void ApplyStyle(string elementName, System.Action<VisualElement> styleCallback)
    {
        var element = FindElementByName(elementName);
        if (element == null) return;
        styleCallback(element);
    }

    static VisualElement FindElementByName(string name)
    {
        var windows = Resources.FindObjectsOfTypeAll<EditorWindow>();
        foreach (var window in windows)
        {
            var root = window.rootVisualElement;
            if (root == null) continue;

            VisualElement element;
            if ((element = root.Q<VisualElement>(name)) != null) return element;
            if ((element = root.Query<VisualElement>().Where(e => e.tooltip == name).First()) != null) return element;
        }
        return null;
    }
}

MainToolbarTimescaleSlider.cs

using UnityEditor.Toolbars;
using UnityEngine;
using UnityEngine.UIElements;

public class MainToolbarTimescaleSlider
{
    const float k_minTimeScale = 0f, k_maxTimeScale = 5f;

    [MainToolbarElement("Timescale/Slider", defaultDockPosition = MainToolbarDockPosition.Middle)]
    public static MainToolbarElement TimeSlider()
    {
        MainToolbarElementStyler.StyleElement<VisualElement>("Timescale/Slider", (element) =>
        {
            element.style.paddingLeft = 10f;
        });

        return new MainToolbarSlider(
            new MainToolbarContent("Time Scale", "Time Scale"),//Content(Text, Tooltip)
            Time.timeScale,//Value
            k_minTimeScale,//Min Value
            k_maxTimeScale,//Max Value
            value => Time.timeScale = value//Action
        );
    }
}