Sometimes I want to test out new colours using online sources like https://coolors.co/palette/a41623-f85e00-ffb563-ffd29d-918450. Is there a way to quickly exchange the colours in my document? I already found swatches. But there you have to manually change each color individually. Is there such a feature like in Powerpoint where you simply click on the colour scheme and it changes automatically like this:
No, there are no automated colour themes like that in Inkscape. However you can kind of do something similar, not quite as automated though.
Assuming you have a bunch of elements of different colours, you can select one, right click it and do Select > Same Fill/stroke etc. Then you can change all the selected objects in one go.