All the widgets allow you to customize the accent color and the background themes (except for the 1Weather classic widget, which allows you to customize only the accent color). Some widgets like the 1W Clock & search, 1Weather Clock(Large), and the 1Weather Tabbed widget also allow you to customize the 'Icon & text color' between black and white.
If you are placing a new 1Weather widget, you can customize these features from the widget settings that pop up immediately.
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 among these background color/theme options.