css

body {
    counter-reset: section;
}

h2 {
    counter-reset: subsection;
}

h2::before {
    counter-increment: section;
    content: "Rozdział " counter(section) ". ";
}

h3::before {
    counter-increment: subsection;
    content: counter(section) "." counter(subsection) ". ";
}

p.definition {
    counter-reset: subsection;
}

p.definition::before {
    counter-increment: subsection;
    content: "Definicja " counter(section) "." counter(subsection) ". ";
    font-weight: bold;
}

p.theorem {
    counter-reset: subsection;
}

p.theorem::before {
    counter-increment: subsection;
    content: "Twierdzenie " counter(section) "." counter(subsection) ". ";
    font-weight: bold;
}