Source

http://www.troopers.fi/~juuba/DCS/customthemes/how-to.txt

Kummeli's Aziz the Combat Fighter theme images and music:

http://www.troopers.fi/~juuba/DCS/customthemes/themes_Aziz_the_Combat_Fighter.zip

Guns 'n Roses Welcome to the Jungle with Finnish Hornet main menu screen and DSC Finland's logo loading screen:
http://www.troopers.fi/~juuba/DCS/customthemes/themes_Hornet-DCSfi-GNRJungle.zip


DCS-Finland logo loading screen, with Battlefeld 1942 theme played by Game Music Collective with Finnish Defense Forces Conscript band:
(Youtube)

http://www.troopers.fi/~juuba/DCS/customthemes/themes_Hornet_BF1942theme.zip

What

a few custom themes for your DCS

How to Install

Unzip the files to "saved games\dcs\mission editor"

Then enable User Theme from DCS Configuration(gear icon) / MISC / Theme

Like so:

How?

Got the info how to do it from:
https://forums.eagle.ru/forum/english/dcs-world-topics/mods-and-apps/229720-how-to-setup-user-theme#post229720

Steps to create one yourself

  1. Go to C:\users\<username>\savedgames\dcs.xxx\missioneditor\
  2. create a folder 'themes' if not exists
  3. create a folder 'main' in 'themes', if not exists
  4. create a folder 'ME' in 'main', if not exists
  5. put your customized wallpaper picture into it (base-menu-window.png)
  6. you can also put customized load picture (loading-window.png) in the same folder
  7. for customized BGM: MainMenuLoop.ogg under Sounds\EditorMusic

My added step

  1. The game will play the MainMenuIn.ogg default audio if not included in user theme.
    Make it an empty file to skip the MainMenuIn-sound from playing.

Make the images the same size as your screen is set at. Also account for space in the main menu for top and botton icons and the left menu structure.