Hexcode for colors
http://www.javascripter.net/faq/rgbtohex.htm
https://www.rapidtables.com/web/color/green-color.html

File location

on mac:
Arduino.app/Contents/Resources/Java/lib/theme/theme.txt 
on Win7: 
C:\Program Files\Arduino\lib\theme\theme.txt
on Linux: 
.../arduino-1.6.3/lib/theme/theme.txt

on Windows 10: when installed as an App, C:\Program Files\WindowsApps\ArduinoLLC.ArduinoIDE_1.8.10.0_x86__mdqgnx93n4wtt\lib\theme\theme.txt, however it does not seem possible to edit it - it can be opened in notepad++ but a save fails.

create a backup of theme.txt before editing
make changes to theme.txt and ensure app is restarted for changes to take effect
make a backup of theme.txt after editing as this will be overwritten when ide is upgraded.

changed console.font to 18
changed console.error.color to #7CFC00 - lawngreen

	# GUI - CONSOLE
	console.font = Monospaced,plain,11
	console.font.macosx = Monaco,plain,18
	console.color = #000000
	console.output.color = #eeeeee
	# original orange #E34C00
	# lime green #BFFF00 
	# lawngreen	#7CFC00
	console.error.color = #7CFC00

changed editor.comment2 to #434F54,bold - dark grey and bold

	# TEXT - COMMENTS
	editor.comment1.style = #434F54,bold
	editor.comment2.style = #434F54,bold

Increased linestatus.font to 14
increased linestatus.height to 30

	# LINE STATUS - editor line number status bar at the bottom of the screen
	linestatus.font	   = SansSerif,plain,14
	linestatus.height  = 30