The default id
generated for the form by the WFFM module is a valid id and meets both HTML4 and HTML5 specifications.
I would recommend not styling based on ID, espcially if you are following module or atomic design patterns. Instead, you should style based on CSS class:
.my-form-style {
/* add your styles */
}
You can then set this on each form in the Custom CSS Class
field on the form item itself. You must set this from the Content Editor, it is not editable from the Form Designer.

NOTE: This is only available in Sitecore MVC implementations.
You can do the same thing for the fields on the form. Again do not style based on ID since it makes your styles inflexible. Instead, use CSS class again:
.my-field {
/* style all fields with this class */
}
.my-form-style .my-field {
/* only style fields in the form where we set custom css class */
}
You can add the field styles in sitecore/System/Modules/Web Forms for Marketers/Settings/Meta data/CSS Classes
using template Extended List Item
, then select the style for each field that you require.

You can find more details in the appearance section of the WFFM documentation.