Userstyle to make Github actually use system fonts (if you have changed them). Probably not exhaustive but all occurrences of sans-serif and monospace I found

gh.css

@-moz-document domain("github.com") {
body,
.markdown-body,
.default-label .sha .ellipses,
.tooltipped::after {
    font-family: sans-serif;
}

pre,
code,
kbk,
tt,
.blob-code-inner,
.blob-num,
.branch-name,
.cm-s-github-light .CodeMirror-lines,
.commit .sha,
.commit .sha-block,
.commit-desc pre,
.commit-ref,
.commit-tease-sha,
.CommunityTemplate-highlight,
.credit-card .cvv,
.credit-card .text,
.default-label .sha,
.export-phrase pre,
.file-editor-textarea,
.file-info,
.gollum-dialog-dialog-body fieldset .code,
.gollum-editor .expanded textarea,
.gollum-editor .gollum-editor-body,
.hook-delivery-guid,
.hook-delivery-response-status,
.input-monospace,
.news .alert .branch-link,
.oauth-app-info-container dl.keys dd,
.tag-references>li.commit,
.two-factor-recovery-codes,
.two-factor-secret,
.uploaded-files .filename {
    font-family: monospace;
}
}