If you tired constantly see steam in dark grey colors then it enough simple fix. 
For this not need download additional programs and enough have only Notepad++ for edit text correctly, 
even not try use ordinary notepad for modify css files. 
Colors by default for Steam client, library and overlay located in Steam / steamui / css / chunk~2dcc5aaf7.css 
or in steamloopback.host / css / chunk~2dcc5aaf7.css, 
for friends list and chat window located in Steam / steamui / css / libraries / libraries~2dcc5aaf7.css, 
or in steamloopback.host / css / libraries / libraries~2dcc5aaf7.css, 
also you can add colors for Steam store and community pages, such as discussions, profile page, 
in Steam / resource / webkit.css, they not located in directory Steam, but in web, 
in community.akamai.steamstatic.com / public / css / ... , 
in community.akamai.steamstatic.com / public / shared / css / ... , 
store.akamai.steamstatic.com / public / css / ... 
in such files as profilev2.css, forums.css, buttons.css, game.css, ... 

Small update, so happened that devs all css code from libraries~2dcc5aaf7.css decide include into chunk~2dcc5aaf7.css
Therefore now enough add custom css only to chunk~2dcc5aaf7 for make changes in local Steam
For make changes on web pages Steam all css need add to webkit.css and every parameter must use "!important" 

For add possibility see different css elements need create in folder Steam for example unknownuser.bat which include it:
start steam.exe ^
-login nickname ^
password ^
-dev ^
-noverifyfiles ^
stop
and run steam through this .bat file
Now you can open DevTools if press F12 or click right key mouse by page and chose option "Open Dev Tools" 
or "Inspect Element". If steam page are game library or overlay then needed window is "DevTools - Steam", 
if it friends list and chat window, or web page, such as store or discussion, or profile page, 
then it called "DevTools - Name Page"
For explain what do next i create small video on youtube and paste here link

https://www.youtube.com/watch?v=mgCOfAAKGtM

added colors for several elements and also it contains clear ingame overlay


