Hi Andre,

In fact, if you understand the workflow of T3, it's quite easy for customization. Let's get started.

1. When Development mode is on
- Your site will run on .less files and your customization with less files will be displayed. All the template .less files are stored in: templates/t3_bs3_blank/less. If you want to customize your template style, you can use the .less files.
2. When Development mode is off
- Your site will run on .css files, the files are compiled from .less files when you implement the "LESS TO CSS" function. We recommend not to use the .css file to customize your template style as each time you compile LESS TO CSS, the .css files will be overridden and your work will be lost.
3. When to use custom.css file and where it is?
- The custom.css file is not inlcuded by default as it's user's custom file so you have to create it by yourselves (templates/t3_bs3_blank/css). The file is not overridden when compile LESS TO CSS.
- The file is for users who are not familiar with LESS.
4. When you should enable and disable development mode?
- ENABLE IT WHEN: you are developing your site with LESS so that the site will run on .less files and you will see how your site is performed with LESS, if it works properly, it will work properly also when dev mode is OFF. Also, you can use firebug to customize your template style easier, you will see the .less rules and where to find the .less file.
- DISABLE IT WHEN: your site is DONE, disable the dev mode, compile LESS TO CSS, enable CSS and JS optmization to improve your site performance.

Thanks for choosing T3 and if you have any further concern, let me know, I am willing to help you and to make sure your site runs properly.

Gardner.