Customization: How to change "Login" text to "Username" on the login / register form

You can use this CSS to change "Login" field label to "Username"

.mh-popup-login-field--login .mh-register-login-field__label:after, .mh-register-field--login .mh-register-login-field__label:after { content: 'Username'; background: #fff!important; padding-right: 40px!important; display: block; top:0; position: absolute; }

.mh-register-login-field__label { position:relative!important; }

.mh-login-register-panel__inner > form .input-group--prepend-icon:first-child > label:before { content: 'Username'!important; display: block!important; }

.mh-login-register-panel__inner > form .input-group--prepend-icon:first-child .input-group__messages { display:none!important; }

In this article you can find information how to add custom CSS to any WordPress Theme: How to add CSS code that is future update compatible