This is very simple, but I like the effect. For some reason, it annoyed me that I would do a Ctrl-Shift-F Search and then click on search content instead of filenames. (I think because at the time I was hard-coding filenames into index-type documents so I would click accidentally into the index document. Nowadays I am more likely to let Dataview generate the index, so it is not so much of an issue). So I made the actual filenames bolder than the search results. Then for consistency, I wanted the filenames in the main Files navigation to be similarly distinguished from the folder names.
.tree-item-inner, .nav-file-title-content {
font-weight:700;
}