This works in D7, but I am sure it transfers to D8.

One approach is to use Views

1. Under Advanced, RELANTIONSHIP add a relationship of flag, in order to get access to the flag link field.

2. Add flag link field, and under Style Settings >> Checkmark customize field HTML >> Select DIV >> type your bootstrap class name.

In your theme's css file add this, ex:

.ddd .flag-action { border: solid green; }
.ddd .flag.unflag-action { border: solid red;  }

This works in D7, but I am sure it transfers to D8.

One approach is to use Views

1. Under Advanced, RELANTIONSHIP add a relationship of flag, in order to get access to the flag link field.

2. In FIELDS, add flag link field, and under Style Settings >> Checkmark customize field HTML >> Select DIV >> type your bootstrap class name.

In your theme's css file add this, ex:

.ddd .flag-action { border: solid green; }
.ddd .flag.unflag-action { border: solid red;  }

This works in D7, but I am sure it transfers to D8.

Click on your flag link field, and under Style Settings >> Checkmark customize field HTML >> Select DIV >> type your bootstrap class name.

In your theme's css file add this, ex:

.ddd .flag-action { border: solid green; }
.ddd .flag.unflag-action { border: solid red;  }

This works in D7, but I am sure it transfers to D8.

One approach is to use Views

1. Under Advanced, RELANTIONSHIP add a relationship of flag, in order to get access to the flag link field.

2. Add flag link field, and under Style Settings >> Checkmark customize field HTML >> Select DIV >> type your bootstrap class name.

In your theme's css file add this, ex:

.ddd .flag-action { border: solid green; }
.ddd .flag.unflag-action { border: solid red;  }