How do you customise bootstrap templates?

Hi there!
My name is Anton, I've launched several services in various areas: logistics, e-commerce, fitness, etc.. All of them failed)

Design is not my strongest skill and to make MVP's, I usually buy bootstrap templates.
Unfortunately, in these templates, there are only examples of the whole page and basic UI elements (buttons, typography, etc). But nothing in the middle. So I have to cut small blocks (widgets, forms, etc) from example pages and then use it my project.

When these blocks are standard - is ok. But when I have to customize some of them, I feel a lot of pain.. It takes ages to make them not to look ugly.

I'm going to make a set of html/css-blocks for myself, that I can easily insert into my pages.

Does anyone have the same problem? How do you solve it?

PS. I'm a newbie here and I'm very glad to join this community!