It is not a plugin but a theme.
to change editor styles I use the funcion add_editor_style
the part of the css file relative to the headers is as follows
html body.mceContentBody h1,
html body.mceContentBody h2,
html body.mceContentBody h3,
html body.mceContentBody h4,
html body.mceContentBody h5,
html body.mceContentBody h6,
.mce-content h1,
.mce-content h2,
.mce-content h3,
.mce-content h4,
.mce-content h5,
.mce-content h6 {
clear: both; }
html body.mceContentBody h1,
.mce-content h1 {
color: #775f38;
font-family: "Roboto", sans-serif;
font-weight: 300;
font-size: 2.3125rem;
line-height: 1.2;
margin: 0 0 20px 0; }
html body.mceContentBody h2,
.mce-content h2 {
color: #775f38;
font-family: "Roboto", sans-serif;
font-weight: 300;
font-size: 1.8rem;
line-height: 1.2;
margin: 0 0 20px 0; }
html body.mceContentBody h3,
.mce-content h3 {
color: #444444;
font-family: "Roboto", sans-serif;
font-weight: 300;
font-size: 20px;
line-height: 1.2;
margin: 0 0 20px 0; }
html body.mceContentBody h4,
.mce-content h4 {
color: #775f38;
font-family: "Roboto", sans-serif;
font-weight: 300;
font-size: 20px;
line-height: 1.2;
margin: 0 0 20px 0; }
html body.mceContentBody h5,
.mce-content h5 {
color: #444444;
font-family: "Roboto", sans-serif;
font-weight: 300;
font-size: 14px;
line-height: 1.2;
margin: 0 0 20px 0; }
html body.mceContentBody h6,
.mce-content h6 {
color: #775f38;
font-family: "Roboto", sans-serif;
font-weight: 300;
font-size: 14px;
line-height: 1.2;
margin: 0 0 20px 0; }