How to add Text Resize button to theme? Whats the best way to add a Text Resize feature in the theme instead of using contrib modules.

I have tried Text Resize but it dosn't work for my custom theme. I told it to resize in entire "body" of my page but it ends up resizing only one paragraph.

are there other themes who have done this?
I would recommend against including this functionality on your site. Re-sizing text is best handled by browsers and assistive technology. Check out this discussion: https://groups.drupal.org/node/85254

The key thing is to use relative CSS units (em and %) when defining font-sizes and typography-related dimensions in your theme.