in Administration > Server settings > Appearance - there's a field for custom CSS:
.compose-form__publish-button-wrapper button {
visibility: hidden;
}
.compose-form__publish-button-wrapper button:before {
visibility: visible;
content: 'Toot!';
display: block;
width: 100%;
padding: 7px;
margin-top:-7px;
border-radius: 3px;
background-color: #595aff;
}
.button.button--block:hover:before {
background-color: #6364ff;
}
this code worked for me like 100% replacement of just text:
.button.button--block {
visibility: hidden;
}
.button.button--block::before {
visibility: visible;
content: 'Beep!';
display: block;
width: 100%;
padding: 7px;
margin-top:-7px;
border-radius: 3px;
background-color: #595aff;
}
}