You could do that for HTML and CSS but I would think you’d rather stage this in a separate development installation via XAMPP/MAMP/WAMP That way, you’d be able to test the theme first and also have a backup of your theme should anything go awry in your server online.
I think you mean Javascript instead of Java, a different animal altogether. Again, I suggest you create/update theme in local server first. WordPress Codex for creating themes – https://codex.wordpress.org/Theme_Development
Thanks also in word press/ buddy press, under the pages tab of editing you can click your page and hit text instead of view and enter your html there.
Good tips, thanks
