1. Install a New Theme on Magento 2.
2. Choose a new preferred Theme and install the Theme.
3. Upload and decompress the theme file into Web-root folder in your site
4. Run the command script via SSH or terminal app of your site: php -f bin/magento setup: upgrade
5. If theme show in list themes. Go to Stores > Configuration > Design > choose design “Your theme”. Save the change. The step will apply the theme for front-end of your store.
6. Run the command script via SSH or terminal php bin/magento
7. Now you can setup the new theme.
8. Go to Admin Panel, click Stores. In the “Settings” section, select “Configuration”.
9. Select “Design” under “General” on the left panel.
10.Open the Theme section
11. In the Design Theme field, select the new Theme as you want.
12. Click Save Config.