modify permanently pocketnews.css

[duns]

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

EDIT

I tried to modify the file (pocketnews.css) leaving unchanged the weight in bytes (629 B) and the date (with the command "touch -r original-file-saved modified-file"), but nothing to do: Cerberus fool notices it.