the following post is inspiring.
I found a solution and the code is posed below.
.CodeMirror pre.CodeMirror-line { line-height: 1.4;/*height of a*/ padding-top: 6px; /*the lower part of height of b*/ padding-bottom: 6px;/*the upper part of height of b*/ }
And the code for adjusting the height of the list item in Preview mode is:
li{ padding-top: 6px; padding-bottom: 6px; line-height: 1.4; }