latex-css/lang/es.css

:lang(es) .theorem::before { content: 'Teorema ' counter(theorem) '. ' !important; }
:lang(es) .lemma::before { content: 'Lema ' counter(theorem) '. ' !important; }
:lang(es) .proof::before { content: 'Demostración. ' attr(title) !important; }
:lang(es) .definition::before { content: 'Definición ' counter(definition) '. ' !important; }
:lang(es) .indent-pars h2 + p,
:lang(es) .indent-pars h3 + p,
:lang(es) .indent-pars h4 + p {
  text-indent: var(--text-indent-size);
}