English at the backend and Other language at the front

Yes – if your chosen theme is translation-ready and you add a Greek translation file.

My theme takes the default wordpress language.
What greek translation file should i add? i tried one but is changing some translations on the back-end too.

What theme are you using (please provide the theme’s download link, if possible)?

i am using reBrand theme.
https://otwthemes.com/demos/rebrand/

I’m sorry but as you are using a commercial theme, you need to seek support from the theme’s vendors. Start by asking them if the theme is translation-ready.