@Antechinus You had a missing } in the file that made it break when using custom. I've fixed it and now it works:
@-moz-document domain("elkarte.net") {
a,a:link,a:visited,a.new_win:link,a.new_win:visited,.lastpost_link>a,.recentpost>strong>a,.inline>strong>a,.keyinfo h5 a,.keyinfo h5 a strong,.lastpost_link>a,.recentpost>strong>a,sup.bbc_footnotes,sup.bbc_footnotes a{color:#a89073}
.bbc_link:link,.bbc_link:visited{color:#a89073;text-decoration:underline;text-decoration-color:#463c31;box-shadow:none}.bbc_link:hover,.bbc_link:focus{text-decoration:underline;text-decoration-color:#85735d;box-shadow:none}.quickbuttons .linklevel1{color:#e9c98c}.quote_button::before{filter:invert(.9)}
.buttonlist li a.linklevel1{text-transform:uppercase;box-shadow:none}#ajax_in_progress{background:#14171b;color:#a89073;border-bottom:1px solid #242a30;box-shadow:0 2px 4px #000}#ajax_in_progress img{float:right;margin:0 1em}::-moz-selection{background:#374450;color:#d9d9d9}::selection{background:#374450;color:#d9d9d9}
#editor_toolbar_container{width:100%;border:0}#postAdditionalOptions,#postAdditionalOptionsNC,#postDraftOptions,.sceditor-container textarea,textarea{background:#101316;color:#9c978c;border:1px solid #293037}
#postAttachment,#postAttachment2,#postDraftOptions .settings dd,#postDraftOptions .settings dt,#postMoreOptions{border-top:0}.drop_area{background-color:#0003;color:#9c978c;border:solid 1px #333}.sceditor-container{position:relative;background:#14171b;border:1px solid #000;border-top:0;border-radius:0 0 4px 4px}
} <------ that last one was missing!
Image of me running custom from my website