If you wish to use any of your predefined Styles that you have set up in the Teams settings (check this guide for more details - LINK), you can easily select them while customizing your design.
To use your Styles in a design:
In your Projects panel, select the design you wish to customize.
Once the design setting window opens, click on "Edit in Design Manager".
Hit the "Choose Style Kit" button at the left bottom side.
Choose the desired style and click "Select", then save the changes in the Design Manager by hitting "Save and Exit".