with css you can center elements by setting the left and right margin to auto. they also have to be block display. example:
.idk{
display: block;
margin: 0px auto; (<- using this syntax, this changes the top & bottom margins to 0, and the left & right to equal amounts, centering stuff.)
}
I'm guessing by "resize borders" you mean changing the thickness and that can be done by:
#whatever{
border: 1px (<- change that) solid #000;
}