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Don't be faraid of Xoops.
Don't be afraid to RTFM either.

That being said...
Each theme has associated css files <style.css, styleNN.css, styleMAC.css>. Edit these files to tweak a themes. They are located in the xoops_root/themes/theme_name/ folder.
As for the main menu. The main menu only displays modules as top level entries. You need a content module to create pages. Getting pages to show up in the main menu is a whole different thread. If you need this search the forum. It has been discussed to death.
JMass