<script>
      function klik(x)
      {
        var c1 = document.getElementById("c1");
        var l1 = document.getElementById("l1");
        var t1 = document.getElementById("t1");
        //
        t1.style.fontSize = c1.value+"%";
        //
        if (x == 1) t1.style.color = "red";
        if (x == 2) t1.style.color = "green";
        if (x == 3) t1.style.color = "blue";
        //
        if (l1.value == 0) t1.style.fontStyle = "normal";
        if (l1.value == 1) t1.style.fontStyle = "italic";
      }
</script>

    <form>
    <p>Podaj rozmiar tekstu w procentach: <input type="text" id="c1" value="150" onkeyup="klik(1)">%</p>

    <p><select id="l1" onclick="klik(1)">
      <option value="0">prosty</option>
      <option value="1">pochylony</option>
    </select></p>
    </form>

    <p>Jakim kolorem sformatować tekst?</p>

    <p><button style="background-color: red;" onclick="klik(1)">-</button>
    <button style="background-color: green;" onclick="klik(2)">-</button>
    <button style="background-color: blue;" onclick="klik(3)">-</button></p>

    <p id="t1">To jest przykład paragrafu</p>