You should take it simple here you go :
first you have the initial position of your text or button :
<div style="background-color:green; height:200px; width:400px; margin:0 0 0 35%;">
<h2> Simple Text </h2>
<div>
<button> Simple Button </button>
</div>
</div>
By adding this css code line to the h2 tag or to the div tag that holds the button tag
style:" text-align:center; "
Finaly The result code will be :
<div style="background-color:green; height:200px; width:400px; margin:0 0 0 35%;">
<h2 style="text-align:center;"> Simple Text </h2> <!-- <<--- here the changes -->
<div style="text-align:center"> <!-- <<--- here the changes -->
<button> Simple Button </button>
</div>
</div>
