Here are the steps:
Choose any page from your multi-page calendar. Right-click the month grid and select Settings. A new window with the grid parameters will open.
Switch to the Month tab. Click Select style to open the grid editor.
Here, you can either choose a template from the collection in Templates, or create your own with the help of the Template Editor. Once done, click OK.
The selected template will be applied to this page. Repeat the steps with other pages to create a unique design for each of them.
However, when you use this way, do not make any changes in the grid template via the Main tab. Otherwise, the new design will be applied to all pages at once.