Editing a Theme in eCom
If you're using a theme you've purchased from the eCom marketplace, it is possible to edit the theme, but doing so will remove the theme from it's update cycle, so you should be sure you're aware of the implications before editing the theme.
What is an "update cycle"?
Themes in eCom can be automatically updated by the theme's developer. Developer's often release updates to themes to fix bugs, improve security or add on functionality. If you edit your theme in eCom, it makes a copy of theme code in your site so you can customize it as you want, but this means any future updates for the theme will not be automatically applied (because you now have a custom theme). You or the theme developer may be able to apply future updates manually, but this depends on the theme developer providing the necessary details for each update, and may require additional costs (for the manual work required to implement the updates).
What if I edit the theme and want to go back to into the update cycle?
It's possible, but it would require uninstalling your customized theme and reinstalling the standard theme from the marketplace, meaning you lose any customization work that had been done to the theme and you'd have to reapply any theme setting adjustments.
Can a theme be modified without editing it?
Some changes can be implemented via custom javascript applied to your eCom account to modify the look of your site. While the theme code won't be modified, with javascript, it is possible to modify the display of your site without editing the theme. Using this method, the code of your site can be modified AFTER the page has loaded. This means your theme code is loading as intended, but is being modified on-the-fly by the javascript code. This allows modifying the display of your website without editing the theme, however this method does have a couple down sides. First, there can be a slight delay after page load before the javascript takes effect (though in most cases, this is negligible), and secondly, this method of development can take longer than directly editing the theme, meaning it can be a little more expensive in initial development costs, but it will allow you to keep your theme in it's update cycle.