Emphasize top-level folders in the file explorer (Obsidian.md snippet). [Not tested on Obsidian 1.0+]

obsidian-highlight-main-folders.css

.nav-folder.mod-root > .nav-folder-children > .nav-folder > .nav-folder-title {
    font-size: .9em;
    padding: .5em .75em;
    background-color: var(--background-secondary);
    color: var(--text-muted);
    margin: 1em 0;
}
.nav-folder.mod-root > .nav-folder-children > .nav-folder > .nav-folder-title:hover {
    background-color: var(--secondary-accent);
}
.nav-folder-children .nav-folder:not(.is-collapsed) .nav-folder-title {
    margin-bottom: .25em;
}
.nav-folder.mod-root > .nav-folder-children > .nav-folder > .nav-folder-title > .nav-folder-collapse-indicator{
    opacity: 0;
}