Alt Data Buttons (described here) can be configured in 2 different ways, depending on the version of the Quik! Forms Engine you are using. For Quik! Forms Engine DLL releases prior to Version 5.5 Release 41 (July 2017), the custom buttons' styles are part of what Quik calls the default styles of the forms. Setting custom css in the HTMLStyles property overrides default styles will result in any default style not getting considered, which fails to show the blue Alt Data buttons. Beginning with Quik! Forms Engine Version 5.5 Release 41, the Alt Data Buttons' styles will not be part of the default styles, meaning that they will be always loaded, no matter if you set the HTMLStyles property or not.
Note: When setting something in the HTMLStyles property, you are responsible for defining all styles you want to be applied to the forms. So if Alt Data buttons' styles are part of the default styles, you will have to put those styles into the HTMLStyles property.