This is what W3C says about it:
Weights: normal | bold | bolder | lighter | 100 | 200 | 300 | 400 | 500 | 600 | 700 | 800 | 900 | inherit
"The 'font-weight' property selects the weight of the font. The values '100' to '900' form an ordered sequence, where each number indicates a weight that is at least as dark as its predecessor. The keyword 'normal' is synonymous with '400', and 'bold' is synonymous with '700'. Keywords other than 'normal' and 'bold' have been shown to be often confused with font names and a numerical scale was therefore chosen for the 9-value list."
So there you go: only "normal" and "bold" are support by the standards, and all the other names are up to the whim of ythe browser renderers. If you want weight "300", then the recommendation is to use weight "300" and not "thin" or "light" or "book" or "demi", as that *will* mean different things to different browsers that use different rendering engines.
I'm sure there will be an outright winner in the end, but the dust has not settled yet on how font weights are handled.
Sorry this became an essay, but I never stop learning how deep and complex this stuff goes.
Let me know if this resolves your issue, i.e. if looking closer, the theme actually is displaying the weight you want. You may need to take a look at the weight values that Canvas uses to select fonts.
Actually, what the hell, here it is:
300 = Thin
300 italic = Thin/Italic
normal = Normal (aka 400)
italic = Italic (aka 400)
bold = Bold (aka 700)
bold italic = Bold/Italic (aka 700)
Looking at this, the weights that Canvas supports look pretty pathetic compared to what Google provides. It just does normal, "thin" (aka Google's "light") and bold. There are another six weights that Canvas does not even recognise. Raise that as a feature request with WooThemes!
It may be useful for me to add an option to this plugin to restrict the loaded font weights to 300/400/700 only. For people using just the stock Canvas and no custom CSS, that option would speed up the loading of the fonts, as it would not be loading weights that are never used.