You can change the title ‘404 Not Found’ in file en_US.po (default one) which is in the languages folder, just need to change the text .You will need to use the editor like POEdit to do this.
http://www.poedit.net/ -Poedit is the popular free editor for creating .po and the corresponding binary .mo files.
You may need to do an update in POEdit to see the changes.
Or
You can change the title in file framework/functions/header-title-area.php line no:158.
Pls use the below custom CSS in themeoptions –
.error404 #title-area {
padding: 45px 0 10px;
background-color: #000;
}
.error404 #title-area h1 {
color: #fff;
}