Hi Muhammad,
If you are using the latest WHMCS version 7.0, then it’s not possible to use the default six theme orderform. This is because, WHMCS changed the design of default orderform for WHMCS version 7.0.
Please learn more from here about the changes: http://docs.whmcs.com/Version_7.0_Release_Notes#Standard_Cart_Order_Form
By the way, you could change the orderform from your WHMCS admin panel Setup > General Settings > Ordering tab
Moreover, you could follow the video tutorials here: https://www.youtube.com/playlist?list=PLfpgUwyOgC7C4wwPhB7jEqSp_esXB31Jb
Also, learn more from here: http://docs.whmcs.com/Standard_Order_Form_Templates
You could also ask them questions about WHMCS throw their forum: https://forum.whmcs.com/forum.php
Even more, if you want to add custom CSS, then add your custom CSS in WHMCS template’s “css/custom.css” file.
Best Regards,
ThemeLooks Support Team