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This will apply your new theme to any new forms you create, but it won’t automatically change the theme for existing forms. To update an existing form with your new theme, locate the desired form and click Edit. Click the toolbar at the top of the form to access the form properties on the left side of the Forms Designer. Click the Style tab, and pick your customized theme from the Theme dropdown box. Custom themes, divided into Application and Designer theme categories, plus the three two Global themes will be listed as choices.
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If you need to make additional changes to your customized theme, again edit the form.css on disk and re-zip the files and then upload the modifications by clicking the icon next to your existing theme. When you update a theme the changes take place immediately and automatically in all forms already using that theme.