Reboot after applying, and If you use rootless Oreo and March security patch you need to follow the instructions in the dialog that shows when you open the theme inside Subs.
Notable changes : Updated and perfected themes for latest WhatsApp and Instagram + other fixes

This theme fully supports Android 7.1.1& it works without TBO Theme ready gapps! Please read the FAQ before installing.

Swift Black Substratum is a theme for the Substratum Theme engine. It comes with a big number of themed apps, great support and frequent updates.

Important notice for 7.1.1 users: The Substratum app will display a warning message about using themes on a 7.1.1 device. You can ignore this warning when using this theme as it have full support for this version.

Themed apps : 

• AOSP Browser
• AOSP Calendar
• AOSP Keyboard
• Calculator
• Chrome
• Chrome Beta
• Chrome Dev
• Contacts
• Dialer
• Documents/Downloads
• Dropbox
• Email (AOSP Email)
• ES File Explorer
• Facebook
• Facebook Messenger
• Gmail
• Google Allo
• Google Assistant
• Google Authenticator
• Google Calendar
• Google Dialer
• Google Docs
• Google Drive
• Google Home
• Google Keyboard
• Google Maps
• Google Messenger
• Google Music
• Google Now
• Google Plus
• GroupMe
• Hangouts
• Inbox
• Instagram
• JuiceSSH
• Keep
• Nova Launcher
• Omni
• Pushbullet
• Root Explorer
• Settings
• Skype
• Slack
• SuperSU
• System UI
• Tumblr
• Viber
• WhatsApp
• XDA Labs
• YouTube
…and more