Choosing an editor
The next step to working on a Shopify theme is to choose an editor.
Edit HTML/CSS page
The Edit HTML/CSS page can be accessed through the Themes page of your Shopify Admin. It allows you to see all of your theme templates and assets, and directly make changes to them.
The Theme gem is a console tool that allows you to sync local theme files with your live Shopify store's theme. The gem runs in a console window and watches for any changes you make to the local copy of a theme. You can use the text editor of your choice.
You can download the gem here.
If you use the TextMate text editor, the TextMate bundle allows you to sync your local theme files with your live shop's theme directly from TextMate.
You can download the bundle here.