获取所用根布局的句柄，然后在其上设置背景色.根布局是您用来调用setContentView的任何内容.
setContentView(R.layout.main);
// Now get a handle to any View contained
// within the main layout you are using
View someView = findViewById(R.id.randomViewInMainLayout);
// Find the root view
View root = someView.getRootView();
// Set the color
root.setBackgroundColor(getResources().getColor(android.R.color.red));