My website has a custom design and it doesn't match up with the themes and styles available for web applications like blogs and forums. I'd like to add a blog to my site, but I don't want my visitors to feel like they've left the site.
Can applications like these be customized? How much does something like that cost? Is it possible to put my site's main menu into the blog - it's CSS-based?
Yes, applications like blogs, forums, e-commerce, and so on can be customized to take on your site's design . . . for the most part. Keep in mind that there are closed source products out there that can not be altered, but I'll discuss open source applications that can adapted for your needs.
First, these applications can be very complex, so depending on the extent of the customizations needed, the cost will vary quite a bit. With that said, following is my choice blog applications taking into consideration functionality, development support and ease of integration.
WordPress Hands down, WordPress is the most versatile blog application available. It's free, it's open source and the enormous community that contributes to it's development have created an infinite list of add-ons (plugins) to extend its functionality.
Per integrating into your site, the best route is to install WordPress as is. Then download and install a theme (free or for purchase) that best matches your current site design and make tweaks from there. This task is best done by a developer that is familiar with WordPress and/or PHP. WordPress is very well built and makes heavy use of CSS. Thanks to the CSS, many changes can essentially be made outside of the application code in an external style sheet.
As a rule of thumb, the less stylized your site is, the easier it will be to customize these applications to match your site design. By stylized I mean the amount of graphics and complexity of layout that come together to create your design. Your site, http://www.ojodeaguapanama.com has a clean design with a lot of white space (not too stylized), so I would say that customizing WordPress to match your site would be pretty straight forward.
And yes, the CSS menu can be added in as well, though this would take several hours of work to get it perfect.
I hope I answered your questions and please let me know if you're looking for recommendations on other web applications. Hint: this forum is a customization of Vanilla by Lussumo.