1. Go to weebly editor and click on "Theme" & "Edit html/css" options


image



2. Select "main.less" and paste the following CSS code at the end as shown in the below screenshot and then save the changes. You can choose any name to save.


iframe.donorbox-widget{

  width: 425px !important;

  margin: auto;

  display: block;

}



3. Publish to see the changes on your site.