Created attachment 156275 [details]
data-warning vs data-warning-symbolic comparison

SUMMARY

Presently, data-warning is a triangle icon with a white exclamation mark in the middle. There is no -symbolic counterpart provided, so requesting `data-warning-symbolic` icon produces just this triangle, filled with white color completely, which isn't particularly useful or descriptive.

STEPS TO REPRODUCE
1. Run the following qml with qmlscene or whatever:

```
import QtQuick 2.15
import org.kde.kirigami 2.20 as Kirigami

Rectangle {
    width: 64 * 2; height: 64
    Rectangle {
        anchors.fill: parent
        color: Kirigami.Theme.backgroundColor
    }
    Row {
        Kirigami.Icon {
            source: "data-warning"
            height: 64; width: 64
        }
        Kirigami.Icon {
            source: "data-warning-symbolic"
            height: 64; width: 64
        }
    }
}
```

OBSERVED RESULT
Solid white triangle on the right.

EXPECTED RESULT
Some kind of usable warning sign. For example, white outline and white "!" in the middle; or white fill and transparent "!".