[Feature Request]: Differentiate header and body content in black theme

[dtbj]

Guidelines

• [X] I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Problem Description

In black theme, the header isn't differentiated and makes FreeTube look less accessible The header isn't separated enough from the body and looks weird.

Proposed Solution

Very Easy two lines of css:

.black {
.topNav {
    border-bottom: 1px solid #ddd;
}
.ft-flex-box.sideNav {
    border-top: 1px solid #ddd;
    border-right: 1px solid #ddd;
}
}

Alternatives Considered

Nothing, just inspect tool for testing.

Issue Labels

ease of use improvement, visual improvement