Not sure how 'effective' that would be...
For one, the "names" of the CSS-based line-items would need to be identical for every page in your .txt list of sites...
Below is a small portion of a .css style sheet open in another window at this very moment...
Code:
.fHL {font-size:12pt; font-family:Arial}
.fBT {font-size:12pt; font-family:Arial; font-weight:bold}
.fFP {font-size: 90%; font-family:ms sans serif,helvetica; font-weight:normal}
.fFL {font-size: 90%; color:#AAA5A3; font-family:ms sans serif,helvetica; font-weight:normal}
.fRC {color:#CC0000; font-size: 11px; font-family:verdana,helvetica; font-weight:bold}
.fCC {font-size: 11px; font-family:arial}
.fHT {font-size: 11px; font-family:arial; font-weight:bold}
.fOH {font-size: 11px; font-family:verdana,helvetica}
.fTT {font-weight:normal; font-size: 90%; font-family:ms sans serif,helvetica}.fSB {font-size:14px; font-family:verdana,helvetica,sans-serif; font-weight:bold; color:#000000}
.fST {font-size:14px; font-family:verdana,helvetica,sans-serif;}
.fLT {font-size: 11px; font-family:verdana,helvetica,sans-serif;}
.fLTW {font-size: 11px; font-family:verdana,helvetica,sans-serif; color: white; text-decoration:none; }
.fLB {font-size: 11px; font-family:verdana,helvetica,sans-serif; font-weight:bold;}
.fTH {font-size:11pt; font-family:verdana,helvetica,sans-serif; font-weight:bold;}
.fMH {font-size: 11px; font-family:verdana,helvetica,sans-serif; font-weight:bold;}
Sure, you can replace "their" style sheet with one of your own, but you need to keep all of the "names" the same for them to be 'applied' to the page...
I can insert that above .css into EVERY page that I visit, but if none of those pages "reference" fCC (number 6 in the above .css), then NOTHING will happen on the page that I inserted that .css into...
Sure, there are "fixes"...
I insert a .css into EVERY page that defines link formatting for visited links, .pdf links, .gif and .jpg links, javascript links, and mailto: links...
And then I disable all .css style sheets...
Or I use a "zap presentational html" bookmarklet that proves quite effective...