After the last update, PokerStars has removed the option to choose the color of the lobby. (currently the lobby appears only in Dark version)
For those who prefer the light view of the lobby:

Instructions:
Spoiler
1. Close PokerStars client.
2. Delete or rename «appdark» folder (for example -appdark) is located into: C:\Program Files (x86) \ PokerStars \ Themes
Note: Once you restart PokerStars your lobby will be overwritten
to avoid this do the following:
3. Right click your PokerStars desktop icon,
➜ Select «Properties» then «Shortcut»
➜ In the target box you will see the line:
“C:\Program Files (x86) \ PokerStars \ PokerStarsUpdate.exe”
➜ Simply delete the word «Update» so its like this:
“C:\Program Files (x86) \ PokerStars \ PokerStars.exe”
➜ Click «Apply», so «Ok»
Note: Use this shortcut every time you launch PokerStars without the update that overwrites your customized lobby.
How to update PokerStars client
1. Close PokerStars client
2. Right-click on your PokerStars desktop icon
➜ select ”Open file location”
3. Run “PokerStarsUpdate.exe” and let it update.
VS