This is quick and dirty, but it should do. At least it does in the test case. Whether it's hand-edited or machine-generated, I think it's safe to assume that we have no CR within a given formatting statement and each and every reference to 'Arial' will be in the context of font formatting (you are not usin this on a tipographer's site, right?)

This is the code:

use strict;

while (<DATA>) {
    s/font-size:\s*10pt/font-size: 18pt/i;
    s/Times New Roman/Helvetica/i;
    print $_;
}

__DATA__
body {
    text-decoration: none;
    text-indent: 0in;
    text-align: left;
    lang: en-US;
    font-weight: normal;
    font-variant: normal;
    color: #000000;
    font-size: 10pt;
    font-style: normal;
    widows: 2;
    font-family: 'Times New Roman';
} 

   <p dir="ltr" style="text-align: justify"><span style="font-weight: 
+bold; lang: en-US; font-size: 11pt; font-family: 'Arial'">Consultancy
+</span></p>
[download]

This prints out:

body {
    text-decoration: none;
    text-indent: 0in;
    text-align: left;
    lang: en-US;
    font-weight: normal;
    font-variant: normal;
    color: #000000;
    font-size: 18pt;
    font-style: normal;
    widows: 2;
    font-family: 'Helvetica';
}

   <p dir="ltr" style="text-align: justify"><span style="font-weight: 
+bold; lang
: en-US; font-size: 11pt; font-family: 'Arial'">Consultancy</span></p>
[download]