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.

Custom CSS

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

Custom Code

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.