Environment

• Package version(s): @blueprintjs/core ^3.22.3
• Browser and OS versions: Mac OS X Chrome 79.0.3945.88

If possible, link to a minimal repro (fork this code sandbox):

Steps to reproduce

Spotted this while building a <ControlGroup> that contained buttons, selects, and some dividers.

<ControlGroup>
    // ...Other controls...
    <Button />
    <Divider />
    <Button />
    // ...Other controls...
</ControlGroup>

Actual behavior

The divider doesn't have its usual 5px padding. Instead, <ControlGroup> overrides with a default -1px padding for all controls.

Expected behavior

The divider should have a 5px padding when inside a `.

Possible solution

// Special case for dividers
.bp3-control-group .bp3-divider {
    margin: 5px;
}

Labeling P3 because it's an easy 1-line fix of custom CSS for application authors. Filing quickly for tracking, but may be able to open a PR soon.