Gradients. A nightmare or a misunderstanding?

[smileBeda]

In the Custom SCSS file:

$enable-gradients: true;
$primary: rgb(0,137,154);
$secondary: rgb(0,66,117);
$gradient: linear-gradient(.25turn, $primary, 50%, $secondary);

• This changes the color of bg-body-tertiary as well, to be a gradient. Why? That color doesn't even use gradients.
• This changes any btn classed element (no matter which), to the gradient too. On top, all buttons lose any sort of hover effect.

I am at a loss to understand two things:

• why?
• and... how do we actually get the buttons to use a gradient (horizontal, just like the code above), while keeping hover effects, without affecting any other element on site?