Don’t worry :)
What you need to do is get access to the WordPress files on your server – as you can’t login to your WordPress dashboard anymore.
So you will need to login to your hosting account (where you website is hosted). Then you need to find something called your ‘file manager’, then you need to find public_html/wp-content/themes/canvas/custom.css. Then follow the instructions above.
Have a go if you get completely stuck – first raise a support ticket with your hosting company and if you are still stuck send me an email with your hosting account details in and I will take a look for you.
Also could you confirm how far did you got with installing the themes and plugins. Did you install canvas, canvas-commerce and woocommerce?