Hello.

Is it possible to use Designer's Mode to work on my custom theme while my community is cloud based and not local on my hosting server ?

I tried to check and did not find a way because as I understand I have to be able to manage the files.

Designer mode is built for you to be able to work with files. As such this is not something you would be able to do on cloud, no

Is there a way for me to work on a Theme from local computer using IDE and then install the theme normally ?

There would be no way in which you could use an IDE to work locally with the cloud platform