Not sure how I didn't fix Lost Cities deck card count ok this one guaranteed does (October 27). Must of used an older version a few days ago. Sorry.
Easiest way without having to add my modifications is just copy and paste it from pastebin which includes my modifications for:
Splendor, Lost Cities, Tichu and Red7 (any others post here and maybe we can figure it out). Not sure how to get in touch with original author cuz I don't have a github account so seems impossible to contact.
https://pastebin.com/rFBxPnd9
1) scroll to bottom of pastebin link till RAW Paste Data
2) select all CTRL-A or MAC CMD-A(?) can't remember
3) copy and paste it into your Profile | Preferences | Advanced
4) after you hit SAVE click to refresh your browser right away
Note: it doesn't make these forums dark mode...that would have to be a phpBB plugin which isn't on here. Everywhere else though from loading screen of games to Play Now area is dark mode.
Now for those who wish to modify and perhaps contribute to new games or ones that aren't working as there are nearly 400 now.
I use firefox but doesn't really matter as concept is pretty much the same.
In the game either playing or watching a game in progress you can right click and choose Inspect
if it has a span id= or div id= then you use #
if it has a class= then use .
example this is lost cities code
<div id="lostcities_wrapper"> (NOT even sure this is needed or not I mean in the CSS part that is.
<span id="deck_count_nbr">0</span>
.bgagame-lostcities #lostcities_wrapper #deck_count_nbr {
color:#FFF;
}
Splendor just found the class and put a . so it looks like in the code class="spl_coinpile_counter" and as far as discovering which one it is that's kinda like trial and error at least for me but I'm getting better at guessing.
.bgagame-splendor .spl_coinpile_counter {
color:#180605;
}
. However playing game for a long period of time during night makes my eye so fatigue. I know that most of the game are designed with the current background so that some components of the game will become hard to see or even invisible under dark mode 
. I was wondering if the developers some day can make implement an optional dark theme for boardgamearena become possible 
