modify permanently pocketnews.css

Today I modified the pocketnews.css (in /news folder) file on PC, but I realized that once the PB was detached from the PC, this stylesheet returned to its original form; it was only possible for me to work on the internal style of a hrss file (and this worked, but it's not a good solution).
How can I change permanently pocketnews.css?

EDIT

Added this two lines:
font-size: 140% !important;
text-align: justify !important;

EDIT

it seemed work, but after restart the original file come back