CustomMargin.cs

// Event with custom elements ( the status bar here has its own colors )        

        private void TextMateInstallationOnAppliedTheme(object sender, TextMate.TextMate.Installation e)
        {
            var panel = this.Find<StackPanel>("StatusBar");
            if (panel == null)
            {
                return;
            }

            if (!e.ApplyBrushAction("statusBar.background", brush => panel.Background = brush))
            {
                panel.Background = Brushes.Purple;
            }

            if (!e.ApplyBrushAction("statusBar.foreground", brush => _statusTextBlock.Foreground = brush))
            {
                _statusTextBlock.Foreground = Brushes.White;
            }

            if (!e.ApplyBrushAction("sideBar.background", brush => _customMargin.BackGroundBrush = brush))
            {
                _customMargin.SetDefaultBackgroundBrush();
            }
        }

textmate.cs

...
                var registryTheme = _textMateRegistry.GetTheme();
                GetOrCreateTransformer().SetTheme(registryTheme);
...

                if (Application.Current?.ApplicationLifetime is IClassicDesktopStyleApplicationLifetime
                    {
                        MainWindow: Window window
                    })
                {
                    _guiColorDictionary = registryTheme.GetGuiColorDictionary();
                    var themeName = theme.GetName();

                    // Applies to the application on a whole.. Some might want to opt out of this for their own setups.
                    if (applyWindowProperties)
                    {
                        if (themeName != null && themeName.ToLower().Contains("light"))
                        {
                            Application.Current.RequestedThemeVariant = ThemeVariant.Light;
                        }
                        else
                        {
                            Application.Current.RequestedThemeVariant = ThemeVariant.Dark;
                        }
                    
                        ApplyBrushAction("editor.background",brush =>window.Background = brush);
                        ApplyBrushAction("editor.foreground",brush =>window.Foreground = brush);
                    }

                    ApplyBrushAction("editor.background",brush =>_editor.Background = brush);
                    ApplyBrushAction("editor.foreground",brush =>_editor.Foreground = brush);
                    
                    if (_defaultSelectionBrush == null)
                    {
                        _defaultSelectionBrush = _editor.TextArea.SelectionBrush;
                    }
                    
                    if (!ApplyBrushAction("editor.selectionBackground",
                            brush => _editor.TextArea.SelectionBrush = brush))
                    {
                        _editor.TextArea.SelectionBrush = _defaultSelectionBrush;
                    }

                    if (!ApplyBrushAction("editor.lineHighlightBackground",
                            brush =>
                            {
                                _editor.TextArea.TextView.CurrentLineBackground = brush;
                                _editor.TextArea.TextView.CurrentLineBorder = new Pen(brush); // Todo: VS Code didn't seem to have a border but it might be nice to have that option. For now just make it the same..
                            }))
                    {
                        _editor.TextArea.TextView.CurrentLineBackground = new SolidColorBrush(CurrentHighlightRendererDefaultBackground);
                        _editor.TextArea.TextView.CurrentLineBorder = new Pen(new SolidColorBrush(CurrentHighlightRendererDefaultBorder));
                    }

                    
                    //Todo: looks like the margin doesn't have a active line highlight, would be a nice addition
                    if (!ApplyBrushAction("editorLineNumber.foreground",
                            brush => _editor.LineNumbersForeground = brush))
                    {
                        _editor.LineNumbersForeground = _editor.TextArea.Foreground;
                    }
                    
                    AppliedTheme?.Invoke(this,this);