Hello,
so far I understand it, the style of the matomo backend can only customize with a plugin/theme. Such plugin/theme is very needed for custom css style. Not a complete new theme. Only a possibility to customize the default style. In my case, there are only few things that will be customize.
Examples:
<div class="card row hoverable">
.card.row.hoverable {
max-height: 500px;
overflow: auto;
}
<textarea name="excludedIpsGlobal" class="materialize-textarea" style="height: 3270px;"></textarea>
.materialize-textarea {
max-height: 500px;
overflow: auto !important;
}