Garmin Fenix 6X Sapphire Map themes

System (the one you didn't list ;) ). Neutral map colors providing the greatest contrast with the course indicator.

Does it mean that The map theme does not affect the routing, only the colors or the contrast?

yes. just how it renders

I use dark, prefer the look plus speed of loading. I have data screens on scroll in my activities and this one seems to load faster, at least for me.

You only get to choose system in the activity settings. System then equals the theme which is chosen as a default in the map settings. 

But what I have in addition to the list in the opening post is "public lands".

I use "popular" for Hiking, running and MTB'ing on trails. (Just to see where others have run/bicycled/walked when I'm in unfamiliar territory).

"None" for all others activities

Maritim because I mostly navigate with it on water.

I use OSM maps. Much better detail than the Garmin maps - and apply a TYP file.

Themes do not work too well then.

Last summer (LATAM region) i was hiking and the map was nearly blank. I realised it was in maritim theme and there was little to show. It changed my whole experience! 

I don't use any, because they only take away information from the map,high contrast for example, just removes information, (in my opinion, and in how I use a map) I'd rather have a different map all together. 

So I use third party maps instead with much more detail.. All OSM based which I and many others contribute to.

Here are a few examples...... 

All these are free..... And optimised for Garmin devices.... Great for hiking etc...

The link below, just choose your tiles and your away, this map is good for street routes, but has contour maps, I'd suggest just uploading the transparent contour map from here....

http://garmin.openstreetmap.nl

Next link below is to Frikart.... Brilliant map, very detailed.. Transparent contour and trail lines/ maps built-in etc.... and transparent trail maps to overlay on other maps.

Also can be used with Automobile routing!... you can use the normally redundant Automobile routing option on the watch. 

http://www.frikart.no/garmin/

This one below is a great map to... You can change the map type to "small" for your watch... Just follow the instructions....
Ps... Remember to rename your map's before you upload them to your watch, as you need to distinguish between them....

www.freizeitkarte-osm.de/.../index.html

An additional link below for biking maps.

https://extract.bbbike.org

Talkie Toaster is an excellent map!!..superb... Get the paid version its so well worth it....good price.... there is a great free version to. 

https://talkytoaster.me.uk/

Paid.... 

Garmin satellite imagery..... 

buy.garmin.com/.../70144

All the best....