Templates require you to use the exact components that the template supports. For example:
Template has gallery where images move from left to right, but you want them to move vertically? Well you are probably back to writing your own CSS AND trying to make it look like the thousands of CSS rules from the template. As soon as you diverge from what a template offers, you are in deep shit written by other people that you now have to understand.
Some of these templates seem to be custom bootstrap themes, in which case you can just customize bootstrap yourself, which you will need to do anyway at some point if you use bootstrap, because otherwise you are stuck on every single bootstrap default or you start hacking over the bootstrap defaults which WILL result in an ugly mess.
Vanilla CSS is easy, flexible and more importantly can be kept to a minimum. No CSS is the best CSS.
So while CSS is fine, there is a problem with default HTML controls / UI components, which is that they mostly don't exist in default HTML. And the one that DO exist suck a lot. This means that every web developer on earth tends to reinvent things like:
- tabs
- menus of various kinds
- dropdowns/combobox with integrated search or with "press key to jump to entry of that letter", etc.
- switches/toggles (that are usable from mobile)
- modal dialogs
- sliders
- date pickers
- search fields
- more advanced tables with sorting/ordering etc.
- ...
I think you get the point.
There is a solution for that problem, which is called "unstyled" or "headless" component libraries. Unlike templates, these libraries provide you with all the JS functionality that you need, usually including responsiveness and even accessability, but they do not contain any CSS (maybe some that is required to make the layout work at all, but without any finetuning/styling). This should usually make it REALLY easy to style them exactly how you want without fighting with pre-existing CSS.
Examples:
https://www.radix-ui.com/
https://react-spectrum.adobe.com/react-aria/index.html (For React i guess)
I'm not saying to you NEED to use one of those libraries, maybe you even should not if you are the beginner, but if you don't want to write everything yourself, i would look into these. And they will integrate with whatever CSS you want. For a simple landing page this is probably massive overkill, but if you want to write APPS, you need this kind of functionality sooner or later.
Or you just copy random JS snippets from Stackoverflow like most people and get things done xD