I compared the CSS's (e.g. http://blah-lbah-blah.com/?bfa_ata_file=css) and found the root cause:
In 3.4.6, near the top of the stylesheet, there is the code missing from (not migrated to) the 3.5.3's stylesheet:
PHP Code:
h1,h2,h3,h4,h5,h6{color:#000;
display:block;
font-smooth:always}
h1{font-size:34px;
line-height:1.2;
margin:0.3em 0 10px;
}
h2{font-size:28px;
line-height:1.3;
margin:1em 0 .2em;
}
h3{font-size:24px;
line-height:1.3;
margin:1em 0 .2em;
}
h4{font-size:19px;
margin:1.33em 0 .2em;
}
h5{font-size:1.3em;
margin:1.67em 0;
font-weight:bold;
}
h6{font-size:1.15em;
margin:1.67em 0;
font-weight:bold;
}
h1 a,h2 a,h1 a:link,h2 a:link,h1 a:visited,h2 a:visited{color:#000;
}
h1 a:active,h2 a:active,h1 a:focus,h2 a:focus,h1 a:hover,h2 a:hover{color:#669900;
}
Otherwise, the stylesheets are very similar, with all my custom modifications/inserts properly automatically migrated from 3.4.6 to 3.5.3.