Add a weighting system to Requirements
CSS and JS files are loaded in the order they are called (with some intelligence based on what requirements are called). When you have multiple modules and your core theme all calling requirements from different places sometimes requirements get called in an order that is not ideal.
My first thought on this was maybe a simple weighting system, so higher numbers float to the top, the call would look something like:
This is following on from the Github issue raised here: https://github.com/silverstripe/silverstripe-framework/issues/6454#issuecomment-271622873