Yes, you can, as long as your end product is a customized implementation of the website template. You will need to register your use of the item and you are allowed to sub-license the customized implementation to your client.

Answer:

Yes, it is permissible to modify items you download on Mellow Elements in order to present them to your clients as a completed End Product. You can modify a theme in order to then use that theme to deliver your client a complete website. However, you cannot modify a template and then sell the template itself as its own product. 

Answer:

No, you do not need to relicense fonts or addons for each project. You only need to register them once for the installation on your device. If registering a font or addon to another computer you use, you need to issue a new license for that installation.

Answer:

Yes, you can.