If you use AutoHotKey then you can bind a shortcut key to the following code to toggle between a day and a night theme.
Path = C:\Users\%A_UserName%\AppData\Local\RStudio-Desktop\monitored\user-settings
FileRead, NewSettings, %Path%\user-settings
If (InStr(NewSettings, "TextMate") == 0) {
NewSettings := StrReplace(NewSettings, "Tomorrow Night Bright", "TextMate")
} Else {
NewSettings := StrReplace(NewSettings, "TextMate", "Tomorrow Night Bright")
}
FileAppend, %NewSettings%, %Path%\NewSettings
FileMove, %Path%\NewSettings, %Path%\user-settings, 1
Return