Yes it is possible to replace the fonts via Custom CSS.
But, the pre-requisite is the availability of Museo font. I have noticed that Museo is a Premium font and is not available in Google Fonts.
Following two points are required,
1. Have you purchased that font for your website ?
2. If yes, than have you been able to successfully load that font in the theme ?
Please confirm the above questions, so i will be able to share a Code Snippet with you.