Fix allowTransparency in lib and demo

Fixes #5558
Fixes #5560

src/browser/CoreBrowserTerminal.ts

@@ -436,6 +436,8 @@ export class CoreBrowserTerminal extends CoreTerminal implements ITerminal {
     this.element.dir = 'ltr';   // xterm.css assumes LTR
     this.element.classList.add('terminal');
     this.element.classList.add('xterm');
+    this.element.classList.toggle('allow-transparency', this.options.allowTransparency);
+    this._register(this.optionsService.onSpecificOptionChange('allowTransparency', value => this.element!.classList.toggle('allow-transparency', value)));
     parent.appendChild(this.element);
 
     // Performance: Use a document fragment to build the terminal

src/browser/Viewport.ts

@@ -71,6 +71,7 @@ export class Viewport extends Disposable {
 
     this._scrollableElement.setScrollDimensions({ height: 0, scrollHeight: 0 });
     this._register(Event.runAndSubscribe(themeService.onChangeColors, () => {
+      element.style.backgroundColor = themeService.colors.background.css;
       this._scrollableElement.getDomNode().style.backgroundColor = themeService.colors.background.css;
     }));
     element.appendChild(this._scrollableElement.getDomNode());