Below is the reply I had prepared before seeing your edit. As for changing the color to "just black", it depends on what you mean. If what you meant was you want headers to be the same color as the rest of the infobox:
.portable-infobox .pi-secondary-header {
background-color: #00101a;
}
If what you meant was that you want both headers and the rest of the infobox to be pure black:
.portable-infobox.pi-background
.portable-infobox .pi-secondary-header {
background-color: #00101a;
}
Based on your reply, I am guessing this is the CSS you want applied to every infobox, correct?
.portable-infobox.type-character .pi-header {
background-color: #89cff0;
}
.portable-infobox ul {
text-align: left;
}
.pi-data {
display: flex;
align-items: center;
}
.pi-title {
text-align: center;
}
Since you want it applied to all infoboxes, you don't need a type or a theme. However, you will need to rewrite it to this:
.portable-infobox.pi-europa .pi-header {
background-color: #89cff0;
}
.portable-infobox.pi-europa ul {
text-align: left;
}
.portable-infobox.pi-europa .pi-data {
display: flex;
align-items: center;
}
.portable-infobox.pi-europa .pi-title {
text-align: center;
}
If you decide to disable the Europa infobox theme, you can just use ".portable-infobox" instead of ".portable-infobox.pi-europa".