You could put your own version of the FormContainerBlock in ~/Views/Shared/ElementBlocks with the extra css class that you need? And disable the defaultForms stylesheet if you need to as well in Form.config in ~/modules\_protected\EPiServer.Forms. Won't that give you enough customization options?

You're partially right, but what I want to do is to customize chosen fields of my form, let's say "Last name" (text input) field needs to be red and "Sex" (select box) field needs to be green. Then I need to:

• create my own classes, one extending TextElementBlock, second extending SelectionElementBlock adding "css class" field to both of them (I need to set it somewhere in Edit Mode)
• create my own views in  ~/Views/Shared/ElementBlocks and copy existing markup of TextElementBlock.ascx and SelectionElementBlock.ascx just to add class="<%: formElement.cssClass %>"

I see some serious issues here: 

I need to create N new classes for each field type, I have to create N new views for each field type copying markup from base views. Now, when EpiServer decides to change anything in their base views/model - I'm screwed. 

IMHO: it's very basic need to be able to set CSS class to a form element (and actually a form container as well) and I was pretty surprised when I didn't find an option to do so