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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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The theme can be selected dynamically via the form Url parameter _themeId. This parameter takes precedence over the theme selected at either the Application or in the Form itself as shown above. Append _themeId can be set to either the name of the theme, "Picasso Professional", as an example, or it can be set to the unique theme Id. You can find the Id via the Copy Link Location on the Edit button or theme name in the Themes tab.

Here is an example using the Just the Iframe share code. The theme is set to "yellow" which is a theme that was uploaded to  by the designer. 

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to the share link of your form/flow or space. There are two types of syntax that can be used. The first type is _themeId=" the name of the theme".  The second type of syntax is extended to use the composite ID: _themeId!userId!tenantID="the name of the theme".

Editing Your Customized Theme 

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.