Making use of CSS vs Sass (SCSS) - base class issues and redundency - css

Question

I'm trying to clean up my CSS to be cleaner by using SCSS.
Standard CSS:
.dark-hr,
.light-hr {
width: 100%;
height: 1px;
margin: 15px 0px;
}
.dark-hr {
background-color: #595959;
}
.light-hr {
background-color: #cccccc;
}
vs SCSS:
.generic-hr {
width: 100%;
height: 1px;
margin: 15px 0px;
}
.dark-hr {
#extend .generic-hr;
background-color: #595959;
}
.light-hr {
#extend .generic-hr;
background-color: #cccccc;
}
Is there any way to avoid creating the 'generic-hr' class that won't be used? I was hoping that some kind of nest would work well.
In this scenario the CSS is definitely way cleaner and more readable than SCSS.
Ideally I would need this to work in SCSS:
.## {
// base class that is not outputted
.dark-hr {
//attributes the extend the base class '.##'
}
.light-hr {
//attributes the extend the base class '.##'
}
}
OUTPUT:
.dark-hr, .light-hr {
//shared attributes defined by '.##'
}
.dark-hr {
// overrides
}
.light-hr {
// overrides
}

Answer 1

What you're wanting to use is an extend class (I call them "silent classes"), which is signified by using a % instead of a ..
hr%base {
width: 100%;
height: 1px;
margin: 15px 0px;
}
.dark-hr {
#extend hr%base;
background-color: #595959;
}
.light-hr {
#extend hr%base;
background-color: #cccccc;
}

Answer 2

Wouldn't you normally do something like this:
.generic-hr {
width: 100%;
height: 1px;
margin: 15px 0px;
&.dark {
background-color: #595959;
}
&.light {
background-color: #cccccc;
}
}

Answer 3

My pattern for this kind of thing is a mixin:
#mixin generic-hr {
width: 100%;
height: 1px;
margin: 15px 0px;
}
.dark-hr {
#include generic-hr;
background-color: #595959;
}
.light-hr {
#include generic-hr;
background-color: #cccccc;
}
This has the added advantage of being extensible, so if you find yourself needing several selectors with really similar properties you can add in variables:
#mixin generic-hr($background-color: transparent) {
width: 100%;
height: 1px;
margin: 15px 0px;
background-color: $background-color;
}
.dark-hr {
#include generic-hr(#595959);
}
.light-hr {
#include generic-hr(#cccccc);
}
.medium-hr {
#include generic-hr(#818181);
}