It's not just because these are "drop-in" files that like these libraries can not be extended or have some variables overridden. For example, TFA says that sim

It's not just because these are "drop-in" files that like these libraries can not be extended or have some variables overridden. For example, TFA says that simple.css uses a variable called "accent" to determine the color of links and buttons. If other styles adopt the same name, then you could just define the "color palette" in one place and have each library working as totally different "design language".