When using the subscription function of Miva, this is the default styling used for the date picker. The CSS is part of /themes/%STORE_ID%/THEME_NAME/extensions.css. If you want to make styling changes, it is recommended you add them to your /themes/%STORE_ID%/THEME_NAME/theme.css file.
If you are using Shadows DE, you can modify the CSS in build/extensions/subscriptions/ and update as part of your build process.