Here is the sample code:

Code:

<script type='text/javascript'>
//<![CDATA
function fontIncrease(){
var obj = document.getElementById("text_Font");
if (obj.style.fontSize==""){obj.style.fontSize="12px"};
if (obj.style.fontSize=="10px"){obj.style.fontSize = "12px";}
else if (obj.style.fontSize=="12px"){obj.style.fontSize = "15px";}
else if (obj.style.fontSize=="15px"){obj.style.fontSize = "17px";}
else if (obj.style.fontSize=="17px"){obj.style.fontSize = "19px";}
else if (obj.style.fontSize=="19px"){obj.style.fontSize = "21px";}
}
function fontDecrease(){
var obj = document.getElementById("text_Font");
if (obj.style.fontSize==""){obj.style.fontSize="12px"};
if (obj.style.fontSize=="21px"){obj.style.fontSize = "19px";}
else if (obj.style.fontSize=="19px"){obj.style.fontSize = "17px";}
else if (obj.style.fontSize=="17px"){obj.style.fontSize = "15px";}
else if (obj.style.fontSize=="15px"){obj.style.fontSize = "12px";}
else if (obj.style.fontSize=="12px"){obj.style.fontSize = "10px";}
}
//]]>
</script>

Who can teach me how to use the function in my forum?

acp>modules>javascript management

make sure javascript is turned on  .. if not  turn it on and save  then

create a new javascript and set
titlle: dont matter
placement in topics


you can change the images to what ever u like

Code:

$(function(){
var minimg = 'http://icons.iconarchive.com/icons/icons8/windows-8/16/Very-Basic-Minus-icon.png';
var maximg = 'http://icons.iconarchive.com/icons/icons8/windows-8/16/Very-Basic-Plus-icon.png';
$('#main-content .post_body').prepend('Font Size: <img id="plfonsize" src="'+maximg+'"><img src="'+minimg+'" id="minfonsize"><br><br> ');
$('#main-content .post_body').css('line-height','initial');
$('#plfonsize').click(function(){
str = parseInt($(this).parent().css('font-size'));
$(this).parent().css('font-size',str+2+"px");
});
$('#minfonsize').click(function(){
str = parseInt($(this).parent().css('font-size'));
$(this).parent().css('font-size',str-2+"px");
});
});

Thank you! My request 's completed.

sorry for jumping in
what version for this Script

Michael_vx wrote:sorry for jumping in
what version for this Script

Good question, and I'd like to know like Michael if this works for phpBB3

That code is for punbb forums 
to get your forum code please make your own threads
Thank you