There's a way to change font size and color in the editor, but it only affects the content inside the editor itself (the content that you add). See this help page for details: http://en.support.wordpress.com/editors/font-size-color/
To change fonts for things like post titles or widget content or titles, you would need to purchase the Custom Design upgrade. There's a font editor built right in that's very easy to use (no coding required), and then you can get help with any coding bits in the CSS Customization forum if you're not sure how to change colors. Just post a link and a description of what you're trying to update. You can preview CSS changes before purchasing, so you might want to try it out once you find a theme you like!
Anyone can review the premium themes. They are in the theme showcase site as well as in your dashboard under Appearance → Themes. You can even preview exactly how the content of your site will look with a premium theme if you click preview from one of them.
Note that the same rules about modifications apply to free and premium themes, you would need a Custom Design upgrade to change the layout of either type of theme (such as things like font color or removing a title like "leave a reply"). So for you, I would recommend looking through the showcase or your Appearance → Themes page to find a theme you like first.
When you say you want to remove the "leave a reply" comment option. Do you want to remove comments all together or just the "leave a reply" title above them. To remove the title, you would use the Custom Design upgrade. To remove comments, see http://en.support.wordpress.com/enable-disable-comments/