wp-includes/blocks/html/editor.css

.block-library-html__edit .block-library-html__preview-overlay{
  height:100%;
  left:0;
  position:absolute;
  top:0;
  width:100%;
}
.block-library-html__edit .block-editor-plain-text{
  background:#fff !important;
  border:1px solid #1e1e1e !important;
  border-radius:2px !important;
  box-shadow:none !important;
  box-sizing:border-box;
  color:#1e1e1e !important;
  direction:ltr;
  font-family:Menlo,Consolas,monaco,monospace !important;
  font-size:16px !important;
  max-height:250px;
  padding:12px !important;
}
@media (min-width:600px){
  .block-library-html__edit .block-editor-plain-text{
    font-size:13px !important;
  }
}
.block-library-html__edit .block-editor-plain-text:focus{
  border-color:var(--wp-admin-theme-color) !important;
  box-shadow:0 0 0 1px var(--wp-admin-theme-color) !important;
  outline:2px solid #0000 !important;
}