CSS file optimization for personal website

For my personal website http://burkard.schemmel.eu/ I need the central CSS file to be optimized. The file should be enhanced to detect mobile devices vs. desktop/laptop computers (CSS Media Queries): One column in the mobile version, 2-3 columns for the tablet/desktop version. Fonts and graphics should not be changed. The website does not use any other tools (pure HTML/CSS)

Scope of work is to re-work 1 CSS file and add the media query into 5 HTML pages, following good practices of responsive web design.