How to decrease the space between widgets?

For the widgets in your sidebar, you can change the spacing by changing this code in your style.css through your WP Dashboard (Appearance > Editor) from This:

.sidebar .widget {
    padding: 30px 30px 20px;
}

to This:

.sidebar .widget {
    padding: 20px 30px 0;
}

Changing the first value will change how large the top padding is, the last value will change how much padding is added to the bottom of each widget.

Glad to help!