-J takes a directory, not a single file name. So your long version doesn't actually work. The correct way would be `dmd -J/home/fantomas/includes`, with `import("default.css")`, `import("icons.data")`, etc. in the D code.
https://run.dlang.io/is/xKQoJZ
So judging from your example, the current -J already does what you want. Closing as WORKSFORME. Feel free to reopen if I'm missing the point.