changing font style and colour default in buttons and forms for interactive pdf

Assuming you mean text fields in a form, the default will be Minion Pro.

If, as I first read your post, you want to change the text you are inputting to a text box (or frame, which some folks also call a field), Derek's answer is correct. 

I know of no way to set a default for this. The response given by @DelBocaVista seems a reasonable workaround.

Derek’s answer is incorrect, unfortunately. While changing the Basic Paragraph style without a document open changes InDesign’s default font for text placed into a document, text entry fields for interactive PDFs, sadly, will still default to InDesign’s default of 12pt Minion Pro. This is especially galling because as Derek did point out, it’s best to use system-safe fonts for these fields, and Minion Pro is anything but that.

The best workaround I’ve found is to keep a library of already formatted interactive elements in my Creative Cloud Library, then drag them into my form and resize as needed. It would be a great timesaver if these attributes could be supported with Object Styles, but that’s the best option I know of at this point.

The thing about this that’s truly frustrating is that creating interactivity elements in Acrobat only makes sense if you never need to revise the layout (in two decades as a designer, I have never had this happen; not even once). Any time you need to update your form, whatever interactive elements you had to create in Acrobat need to be created all over again from scratch. Acrobat is an output tool. Any time you’re doing major edits in Acrobat, there’s a good chance you’re creating a massive headache for yourself down the road.