For the required validation you are getting for the file field you should be able to go into the server behavior and find the server validations, then remove the one that is applied to the file field. This way you will not get the error if you leave the field blank.
If you need to add in some css for the form you should be able to do so by editing the css that can be found in CSSFormBuilder with the name of the style you chose.
I'm not sure why you are getting the error regarding the preg_match function. The file that was generated might be improperly formed. You can try deleting or making a backup of this file and adding a new form to a new page to have it be recreated. If you still have issues with this please let us know and include any other info about OS, DW version and installed version of CSS Form Builder.