You can not really edit t he html/markup in Telligent Community 6.x (other than the wrapper classes for widgets headers/footers etc).
Themes in 6.x are xml based and stored in the database and therefore there are no physical pages or page layouts to edit.
You need to implement the design working with the markup you have and apply css to it to achieve the layout/design you want. You can select around 18 different page layouts OOTB to use as a basis for each page and can configure the widget borders/headers etc to achieve different layouts.
Essentially to create a new theme you need to:
- Go in the control panel and copy either the fiji or enterprise theme and save it as your new theme name
- Open up screen.css and make changes here for your theme
- Create and upload any images you wish to use into the supplementary files section in the control panel under the theme management area
- Create custom widgets or modify OOTB widgets for your needs
- Export your theme and import it into different environments (Test / Staging etc)