Hey, @shamseer. To adjust themes, first, open your configuration.yaml. You might need to install File Editor (Configurator) first. Then add this to it:
frontend:
themes: !include themes.yaml
Click on save. Make a new file called themes.yaml. Add this to it:
Rounded theme:
ha-card-border-radius: '20px'
Click on save. Restart Home Assistant. Click on your profile icon, and under themes choose Rounded theme. You should have all of the cards rounded.