there are more options.
1. open e107_core/templates/header_template.php - find correct place - NOT RECOMMENDED, just for testing purpose, it will be overriden with next update. Some people say you can copy this file to your theme and it will woek, but I didn't test this.
2. use theme_head() function in theme.php with echo () function, but correct unescaping '' and "" is important
3. use e107::js() / e107::css() in theme.php , if you use inline parameter, correct unescaping '' and "" is important too
3. create plugin for this using e_header.php with e107::js() and e107::css() functionality
New version of metatag plugin is tested now here... some messages can be see from time to time :)
There is new theme mdbootstrap in my github repository for anybody who want to learn new way how to build e107 theme. It's built with focusing on step by step. Anybody who have interest, just let know about you and start ask questions while I am on the beginning.
For download something, you need to subscribe class Real e107 user in your profile. (if you didn't get message on homepage)