Hi @antoineselim, please note that many of the themes are incompatible with your installation due to being obsolete or outdated.
However, edX provides some official themes by default. You can these at "/opt/bitnami/apps/edx/edx-platform/themes".
As for other themes (e.g. advanced functionality), unfortunately there isn't any edX theme marketplace available, but pages such as ThemeX offer some for free.