If you wish to hide the ProtonMail logo, you can do so with this custom CSS code:
.headerSecuredDesktop-logo {
display: none
}
.headerSecuredDesktop-logo {
display: none
}

h1 {
font-size:24px;
}
h2 {
font-size:22px;
}
h3 {
font-size:20px;
}
h4 {
font-size:16px;
}
h5 {
font-size:14px;
}
h5,h6 {
font-weight:700;
}
h6 {
font-size:12px;
}
.spinner1,.spinner2 {
background-color:#9397cd;
}
#et-info-phone { display:none !important }