SharePoint Templates

Tim Hunter Can you let us know what version of SharePoint are you using? The ability and level of customization available to you varies by  the version  of SharePoint you are using.

Intranet Templates are expensive, because branding SharePoint is A LOT of work for various reasons. If you are on-premises, you have the ability to create whatever HTML/CSS (use bootstrap if you want) in house, but I would highly advise against it especially if you do not have a lot of experience in doing it. It involves custom master pages, page layouts and working with SharePoint HTML (which, to be frank, isn't as optimized or simple as it should be).  SharePoint is very easy to break, when guidance isn't followed correctly. There are a lot of ways to do something, and many of them have negative impacts on your environment.

In any case, take a look at the branding guidelines. This will walk you through the options available to you and will also provide links to all of the information you would need in regards to branding a SharePoint environment.

SPO on the top and on-premises at the bottom.
https://docs.microsoft.com/en-us/sharepoint/dev/scenario-guidance/branding