There is a border on TWebPageControl that I cant get rid of. (shown in the red squares in the picture)

To the right you see the css it seems there is border declared. I am newbie with css.

Border Style is set to none

I want to have everything as a plain black... only letters in white

This border is the TWebTabSheetBorder.
You can override this with CSS by adding in the project HTML:

  .sheetborder { border: none; !important;}

and setting for each TWebTabSheet.ElementClassName = 'sheetborder'