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() {
        var icon = EditorGUIUtility.IconContent("SettingsIcon").image as Texture2D;
        var content = new MainToolbarContent(icon);
        return new MainToolbarButton(content, () => { SettingsService.OpenProjectSettings(); });
    }

    [MainToolbarElement("Timescale/Reset", defaultDockPosition = MainToolbarDockPosition.Middle)]
    public static MainToolbarElement ResetTimeScaleButton() {
        var icon = EditorGUIUtility.IconContent("Refresh").image as Texture2D;
        var content = new MainToolbarContent(icon, "Reset");
        var button = new MainToolbarButton(content, () => {
            Time.timeScale = 1f;
            MainToolbar.Refresh("Timescale/Slider");
        });
        
        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) {
                image.style.width = 12f;
                image.style.height = 12f;
            }
        });
        
        return button;
    }
}

MainToolbarElementStyler.cs

using UnityEditor;
using UnityEngine;
using UnityEngine.UIElements;
using UnityUtils; // https://github.com/adammyhre/Unity-Utils

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 = null;

                if (element is T typedElement) {
                    targetElement = typedElement;
                } else {
                    targetElement = element.Query<T>().First();
                }

                if (targetElement != null) {
                    styleAction(targetElement);
                }
            });
        };
    }

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

MainToolbarTimescaleSlider.cs

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

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

    [MainToolbarElement("Timescale/Slider", defaultDockPosition = MainToolbarDockPosition.Middle)]
    public static MainToolbarElement TimeSlider() {
        var content = new MainToolbarContent("Time Scale", "Time Scale");
        var slider = new MainToolbarSlider(content, Time.timeScale, k_minTimeScale, k_maxTimeScale, OnSliderValueChanged);
        
        slider.populateContextMenu = (menu) => {
            menu.AppendAction("Reset", _ => {
                Time.timeScale = 1f;
                MainToolbar.Refresh("Timescale/Slider");
            });
        };
        
        MainToolbarElementStyler.StyleElement<VisualElement>("Timescale/Slider", (element) => {
            element.style.paddingLeft = 10f;
        });
        
        return slider;
    }

    static void OnSliderValueChanged(float newValue) {
        Time.timeScale = newValue;
    }
}

Regarding the MainToolbarElementStyler.StyleElement method, if the element name contains spaces, unfortunately it will not work.

  In principle, it worked quite well.
  
    static VisualElement FindElementByName(string name)
    {
        var nameWithoutSpace = name.Replace(" ", "");
        var windows = Resources.FindObjectsOfTypeAll<EditorWindow>();
        foreach (var window in windows)
        {
            var root = window.rootVisualElement;
            if (root == null) continue;

            VisualElement element;
            if ((element = root.FindElementByName(name)) != null) return element;
            if ((element = root.FindElementByName(nameWithoutSpace)) != null) return element;
            if ((element = root.FindElementByTooltip(name)) != null) return element;
        }

        return null;
    }

And one more thing that wasn't in the video, if for example you need to combine a slider and a button into one tool, then you need to return not MainToolbarElement but IEnumerable'<'MainToolbarElement> where to each yield return add the desired element