After some experimenting and turning GeoServer's logging level up to GEOTOOLS_DEVELOPER it turns out that GeoServer didn't understand your SLD as it didn't contain any schema locations. So changing your SLD to start with the following worked.
<StyledLayerDescriptor version="1.0.0" xmlns="http://www.opengis.net/sld" xmlns:ogc="http:...
For anyone else who may run into the same issue, I've found this works:
background-color: rgba(230, 230, 230,0.7);
I had originally been calling it popup in html but it had to be the above in order to change the displaying properties
I found out, that I missed the label-TAG with PropertyName:
As far as I know, the overall size of the legend cannot be forced, and there is no word-wrapping logic (only the labelling engine in the map supports word wrapping, the legend graphic builder is an entirely different animal).
Thank you @nmtoken. This tip made it for me.
I'm using a WMS with elevation-raster provided from HEXAGON AB by their Danish content programme. The service has 4 layers in it providing different data sources. Each with 12 alternaive styling formats besides "Default". If you just throw in the standard WMS URL into QGIS it will show the Default style, ...
It all depends what layers are available at particular WMS service. Since you didn't specify which specific service you are using, it's impossible to give definitive answer. But generally you start with the GetCapabilities request to your WMS service and check what layers are available.
For example, for one of geonorge.no WMTS services request would be (gkt ...