There is no specific example. The HTML5 Report VIewer's template (HTML/CSS/JS) is available for modifications by directly changing the default settings or by using external HTML/CSS/JS to affect the HTML elements rendered for the viewer.
You can use the browser's console to expected the viewer's HTML elements and their attributes, which styles and how are applied and then to modify the template.
Note that changes in the default settings of the viewer will cause you to upgrade manually the template files in future. Thus if you need customized parameters area, you may prefer to create entirely custom UI that updates the viewer's reportSource - How To: Pass Values to Report Parameters
I hope this information is helpful.
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