Please follow the step to create new custom template for category and content page.
Step 1: Create one .html file with the file name start with underscore(_). For example _customPage.html , _customCategory.html
Step 2: Copy the existing template code from bigcommerce and paste into _customPage.html file.
Step 3: Upload _customPage.html inside “/dev/template”.
Step 4: Now jump into the admin panel of your bigcommerce and go to “web pages” under “Web Content”.
Step 5: Edit the page you want assign custom template.
Step 6: Now go to at the bottom of the page and click on drop down box correspondent to “Template layout file” and select your custom template file.
For category follow the same step just make sure you copy default template category.html file rest step remain the same.