Yeah, Google browser and other browsers have a zoom feature to make text look bigger that the website encoding style.
I tested all the browsers and sizes, seems to work better, you can visit:
http://whois.domaintools.com/eevblog.com under Site Profile to see your SEO score. It has to do how your content is displayed - I see 78% which is good.
Yes WP does allow to change the theme and everything down to the text. Check out WP Admin > Appearance > Editor >
Most of this is written in HTML, PHP, W3C CSS.
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You must be seeing something different to me then, attached is what I see.
I don't see any "really large" text in the headines, and I don't see the dangling text.
No WP does not allow people to change the theme as far as I am aware.
Dave.