Go to your C2 folder /exporters/html5/node-webkit. Find package.json and open it with a text editor. There you'll see a number of options for your exported project, like window width, height, position, etc.
If you just want to remove the window frame (toolbar, border, etc.), add the line "frame": false to the bottom of the "window" list and save the file. Be sure to add a comma to the end of the preceding line.
There's also kiosk mode, which I really like as way to give games that professional feel. This is done by adding "kiosk": true to the list. It'll set the game to borderless full-screen mode and remove the option to un-maximise it. Just be sure to include a quit function in your game (Browser object -> close).
You can also play with package-preview.json for quicker results without having to do a full export each time.