In Bo Tu WinCC, you can change the background color and grid transparency of the screen window by the following steps:

1. Open the WinCC project and select the screen window to modify in the screen editor.

2. In the properties window of the screen window, select the "Background" tab.

3. In the Background tab, you can choose different background colors or background pictures, and you can also choose whether to display grid lines.

4. If you need to adjust the grid transparency, you can set it in the grid tab. In the Grid tab, you can choose the color and transparency of the grid lines, and you can also choose whether to display the grid lines.

5. After setting, save the changes and preview the screen window to see if the effect meets the requirements.