Regular font icons, such as ".C" and ".H", indicate files which are recognized as build target source code or headers located in the include path of a project.
Conversely, files shown as outlined font icons are excluded from build target, or out of the include path.
For example, if only the files in the /src folder of a project are specified as build targets, the icons of files outside the /src folder have outlined font. Therefore, they are recognized as being excluded from the build target.
By moving source files into /src, the icon will change to normal, and then be accepted as build target.
If you wish to configure a build target folder, refer to the following FAQ page.