gitea_custom/templates/custom/body_outer_pre.tmpl

{{ if .IsSigned }}
  {{ if and (ne .SignedUser.Theme "gitea") (ne .SignedUser.Theme "arc-green") }}
    <link rel="stylesheet" href="/assets/my_css/base/gitea/{{.SignedUser.Theme}}.css">
  {{end}}
{{ else if and (ne DefaultTheme "gitea") (ne DefaultTheme "arc-green") }}
  <link rel="stylesheet" href="/assets/my_css/base/gitea/{{DefaultTheme}}.css">
{{end}}