You can edit the form of the Contact Solution pack with CSS Form Builder, you would just need to select the form in DW and insert your new CSS Form Builder form. There is more information about updating your existing forms to be CSS Form Builder forms in the Getting Started Guide that can be found in the CSS Form Builder support page, here is a direct link:
Nothing at all happens when I upload them. I can fill in the details but since I have not been able to configure a POST action, the server does not know what to do with the form.
As stated - I uploaded a Contact Solution Pack form and this works because there is configuration and I can specify an email address to send the data to. This seems to be where I am having problems when it comes to CSS Form Builder - There is no configuration for an email address to send or syntax for how it should be added to the form.
I tried the GSG that you kindly linked (above) but that does not mention how to go about editing forms from Contact Solution Pack.
Step by step deployment guide would help.
Thanks for responding thus far.