Offline map is available only in GPX Viewer PRO. So this how to won't work in free version of GPX Viewer.

Currently it is possible only to modify existing offline map styles available in GPX Viewer PRO and add new offline map styles based on these existing styles. Some styles have hillshading and elevation contours to be shown on the offline map. All styles support 3D buildings rendering and showing transit lines. Refill style has multiple color variants to experiment with.

Below are step by step instructions how to add offline map style.

1. First off you need to start GPX Viewer PRO.
2. Open left main menu, then tap on Settings button. In settings screen tap on Maps button and then Offline map style manager.
3. In Offline map style manager tap on + button in bottom right corner.
4. Tap on Offline map style option which will show you dialog with available offline map styles. Tap on some style you like (we have selected Refill in video below).
5. To recognize your style in Offline map dialog name it by tapping on Name option (we have named it Dark).
6. If you have selected Refill as base style you can change color theme by tapping on Color theme.
7. You can also adjust other options (we have enabled transit lines).
8. Go back to main map screen by tapping back button multiple times.
9. In top toolbar tap on map button without globe. If you hidden this button previously tap on … button in top right corner in this top toolbar and from menu select Offline map. If you have also hidden this button, open left main menu, then tap on Settings button. In settings screen tap on Maps button and then Offline map.
10. In shown Offline map dialog select style you have added and tap on OK button.

Below is video showing following instructions above: