If you have already placed a 1Weather widget, you can access the theming features by going into the widget settings. Open 1Weather -> Menu -> Settings -> Widgets to access the widget settings.
You can change the accent color of the widget to a few pre-set colors. Or you can select the color picker to select any color of your choice to make your widgets beautiful.
The widgets also offer customizable background colors/themes. You may choose between light, dark, live weather, and transparent color/theme. Note that some of these widgets allow you to choose only a few from these background color/theme options.