The reason you have a SharePoint site is that you and your team are using it, and a big part of using a site is being able to read it.
The following common checkpoints for websites might apply to your site look choices or perhaps your content on the team site pages as well:
Make sure there is a strong contrast between the background colors and the text. Dark text on a white background is generally considered the easiest to read. The second best is very light text on a very dark background.
One area of SharePoint that this has been a problem with in the past is the Quick Launch menu or left navigation area, where the contrast between the background and links isn’t distinct enough. Be careful with your selections. Even if red and green are holiday colors, red text on a green background isn’t very readable.
Choose a font that’s simple and easy to read. No matter that the Chiller font looks cool at Halloween, a whole page of Chiller will have your users running for the door, or at the very least not reading your site.
Use only a few colors. Even though it seems like the theme palette has a lot of color options, many of them are similar in hue. Using the entire rainbow makes it hard for your users to focus on what’s important.
Make link colors obvious. If the text is black and links are navy blue or brown, it becomes difficult to identify them.
Make the followed (or visited) link color different enough from the unvisited link color. A red hyperlink that changes to maroon when visited may not be enough of a visual cue to users that they’ve followed that link.
Microsoft took the liberty of helping you choose decent combinations when it created the out-of-the-box looks. Some of the looks are downright scary, though. Try changing your site to the Sea Monster look and see how your users run away, or maybe run to you, screaming.