Here is a full review by my usual scheme.
Color scheme is pleasant and the site seems easy to browse and navigate so first impressions are quite decent. But there are a few issues that stand out where improvements could be made:
1. The colors of the logo don't quite seem to fit the background they're on. There is this clash between red and blue and green seems a little too bright given the background. Colors used in the logo also don't seem to match the color scheme of the site enough. The red used is of a different nuance than the red of site links while green and orange aren't anywhere else in the general design.
Logo doesn't necessarily have to match the color scheme of the site entirely of course. There can always be different colors in the detail, but I think it does help if it fits in well.
Also, albeit this is a more minor complaint, I'm not sure if the font of "today" really looks better as it is as compared to matching the font of "Theme". Maybe it could look overall neater if they were same font. It might be something to experiment with further.
2. The header background uses a different kind of blue than the rest of the site, but the sudden switch to it from the rest of the site makes me question the prudence of that. Maybe it would be better for the header to have same background?
3. In the sidebar a second "Advertise here" box isn't the same size as the above one, but if two boxes of that size were put next to each other they still wouldn't align with the above box. This is also a rather minor complaint, but alignment always contributes to a more rounded and neater look.
4. "Submitted Themes" section of the sidebar seems a little cluttered and crammed. "Submit a theme" link is pushed right next to the section title with very little spacing and there also doesn't seem to be enough spacing between the two items below (SnapMagazine and Deshawn). The ad box then appears immediately below, also with too little spacing. So overall these items seem like colliding with each other. Spacing does wonders in web design as it's a quick way to make things look cleaner and more readable.
5. The said big ad box might look better centered rather than left aligned.
6. There is a google adsense banner at the top of the posts in full view. I usually don't quite think this is a good idea as it seems a little too intrusive. Putting it below instead and adding just a little more space between the article ending and the ad probably wouldn't fail to give ads almost just as much attention as if it was on the top, at least for those who read the article, but would be far more user friendly.
All this said it's not a bad design at all in general. It flows well, there are some nice touches (the way number of comments is displayed, a nice AJAX tag/archives/categories box) and all those images within posts gives it some extra appeal. However applying the above suggestions might give it some extra polish that may make a positive difference.
Design Rating: 8/10
With a very good domain name, ThemeToday.com pretty much spells out the focus in its very name so that's an immediate big plus. The site and it's content doesn't ruin it either as just at a glance it's obvious that it's all about design and theming. Even the ads are exactly relevant to it.
Focus Rating: 10/10
There are plenty of content entries most of which are overviews of themes, vectors and other design resources filled with images for preview, so there's definitely useful stuff to find. Some entries are odd fits, like the one about PHP Dir Submit which doesn't really seem exactly fit to site themeing although given the expected site audience it's probably of interest.
Not much else to complain about here.
Content Rating: 10/10
A lot of useful content and decent designed coupled with above suggestions would make this an excellent site.
With regards to what it would be worth for sale I have no experience in appraisal, but there are two ways in which you can with some reliability begin your value estimate. One is Estibot which seems like a pretty good estimation tool and another is a common method of just summing up your yearly earnings. If the two end up matching it might give you a pretty good picture of what you could get. Estibot gives it $290 for the domain, and $20 000 considering traffic, but couldn't verify that (with its verify feature) and it might be a little over the top, depending on what your traffic actually is.