There's an extension called Stylus (
Chrome,
Firefox) which makes it easy to customise the layout of sites and even individual pages if you have a bit of CSS knowledge. It shouldn't bog down your browser at all as it simply does a pattern match on the current URL and injects additional styles into the page. The font can be improved in most places by "simply" adding these styles for
forums.tomshardware.com:
CSS:
html {
font-weight: normal;
}
Some things won't change (eg. thread titles in the thread list) a they have more specific styles applied which need to be overridden, but it would be possible to change those too with a bit more work. Of course it would be nice if the font weight wasn't light to begin with, but this is a decent workaround for now I think
Can't do much about differences between browsers. It's mostly up to the browser/OS how it renders a font. CSS can only influence it.
I'm surprised this ever worked. Browsers usually squash redundant space. Maybe the old forum replaced multiple consecutive spaces with non-breaking spaces (which are exempt from squashing), but if anything that's unusual, not expected.