i am a beginner to linux and every single time i log in to my computer the theme changes to adwaita personalized
i want it to be zorin blue-dark
Have you installed any software or made other changes to customize using online guides since you installed Zorin OS?
yeah i think i have installed a few extensions and followed a few guides from this forum
I would think Forum Specific guides would not likely add anything that would disable the Zorin Theme... I would hope anyway... 
You might check your ~/.config directory for anything directing a theme change.
We can test that by making a back up of your config directory by running this command in terminal: mv ~/.config ~/.config-bk
Then log out and in (NOTE that all your personalizations will be reset to default) and testing if you can set the theme and have it persist through reboots or logins.
If it does, then you can presume it was a wayward config. If it does not, you can remove the new ~/.config directory and rename the backup to restore it: mv ~/.config-bk ~/.config
If able to remember, which extensions did you install?
the command worked
I guess I have to check what extensions I install more carefully😅