Make default map background transparent

[julcnx]

Description

Currently, the map background is set to black which makes it difficult to view OSM data when:

• "none" background layer is selected
• a transparent custom layer with mostly dark features is imported

I believe in terms of readability and UX, it would be better to have a transparent background as default:

.ideditor .main-map {
  /* background: #000; */
  background: transparent;  
}

Screenshots

Examples:

[anniemichel]

Would I be able to take this issue? Are you aware of which area in the code this issue lies?