Adding custom css or custom code to the template can be done without tempering with template files. The template has this feature built right in. You can go to your template, and select the tab cutom css or custom code.
Adding custom css can be done for various layouts:
Global - css places here is active with any viewport
Desktop - css is only active in desktop mode
Mobile - css is only active in mobile mode
Adding custom code can be done in the tab custom code
You can add
Custom head scripts, to be placed inside the <head></head> section
Custom begin body: scripts will be places right after the <body> section
Custom end body: scripts will be places right before the </body> section
Google analytics: here you can paste the complete embed for your google analytics code. It will also be placed at the end of the <body> tag.