There is a standard for CSS variables and a very functional subset of their functionality can be compiled for older browsers at build time: http://cssnext.io, yet as far as I can tell, all of these component frameworks impose some other nonstandard compiles-to-CSS language or one of many possible APIs for writing inline styles.
Inline styles seem to be a trend at the moment but I can only assume the people driving this movement haven't had the experience of working with 3rd party components (which you may or may not have the source for) and needing to alter something the designers didn't expose and simply having no simple options because the damn inline style can't be overridden even by !important. Now you've got to fork whole the f'ing component, possibly multiple, each of which potentially coming with yet another different compiles-to-CSS toolchain all because you wanted to add a single line of CSS.
Admittedly, antd doesn't use inline styles si the above doesn't apply here, but having to maintain a custom builder for antd in order to change some fonts and a few colors was a frustrating experience. Not sure what a good solution is until CSS variables get more browser adoption, but it'd be great if we could collectively start standardizing on standards.