Tested on version 0.119.4 might get improved later:
After adding files starting a new game or loading a savegame might take a while, be patient and wait for the game to load untill the Startbutton in the right corner appears.

For every typ of customization we will be in \Steam\steamapps\common\Occupy Mars The Game\OccupyMars_Data\StreamingAssets
There you will find
Boardicons
Patches
Music

You can add your own patch to your character which also shows up on the Tablet.
Simply go to your gamefolder (\Occupy Mars The Game\OccupyMars_Data\StreamingAssets\Patches)
inside this folder you can add any icon you want as long it has a filesize of 512x512 and has the ending .png

Once done you can go ingame select Profile and change the icon.

Ingame you can create signs once you unlocked the personalization in one of the Techtrees, then you can print them with the 3D printer.
For decoration items you need the handdrill which also can be printed
Take out the handdrill, select with the mousewheel what you want to do (for boards choose the board) slap it where you want (only woks inside). Then you can edit that, choose a test or change the icon.
For custom icons: \Occupy Mars The Game\OccupyMars_Data\StreamingAssets\Boardicons
Put whatever picture you want as long its an .png

Once you've unlocked the Radio in the Techtree you can print it and place it where you want inside your base
To add music drop your .mp3 files in here: \Occupy Mars The Game\OccupyMars_Data\StreamingAssets\Music
Ingame you then click on the radio, your files should show on the left side on the radio, doubleclick them to add and press play to listen to your favourite music.

Additional note: Only .MP3 will work and the music will only be where the radio is, havent tested multiple radios yet.