css/ - goes under $FORGEJO_CUSTOM/public/assets
fonts/ - under $FORGEJO_CUSTOM/public/assets
templates/ - under $FORGEJO_CUSTOM/

/var/lib/forgejo/custom
└── public
    └── assets
        ├── css
        │   └── theme-0xf0xx0.css
        └── fonts
            ├── FiraCode-Regular.woff2
            ├── IosevkaF0xf0rge-Bold.woff2
            ├── IosevkaF0xf0rge-BoldOblique.woff2
            ├── IosevkaF0xf0rge-Oblique.woff2
            └── IosevkaF0xf0rge-Regular.woff2