This somewhat depends on what platform you are on. The most common way to do this is by printing ANSI escape sequences. For a simple example, here's some Python code from the Blender build scripts:
class bcolors:
HEADER = '\033[95m'
OKBLUE = '\033[94m'
OKCYAN = '\033[96m'
OKGREEN = '\033[92m'
WARNING = '\033[93m'
FAIL = '\033[91m'
ENDC = '\033[0m'
BOLD = '\033[1m'
UNDERLINE = '\033[4m'
To use code like this, you can do something like:
print(bcolors.WARNING + "Warning: No active frommets remain. Continue?" + bcolors.ENDC)
Or, with Python 3.6+:
print(f"{bcolors.WARNING}Warning: No active frommets remain. Continue?{bcolors.ENDC}")
This will work on unixes including OS X, Linux and Windows (provided you use ANSICON, or in Windows 10 provided you enable VT100 emulation). There are ANSI codes for setting the color, moving the cursor, and more.
If you are going to get complicated with this (and it sounds like you are if you are writing a game), you should look into the "curses" module, which handles a lot of the complicated parts of this for you. The Python Curses HowTO is a good introduction.
If you are not using extended ASCII (i.e., not on a PC), you are stuck with the ASCII characters below 127, and '#' or '@' is probably your best bet for a block. If you can ensure your terminal is using a IBM extended ASCII character set, you have many more options. Characters 176, 177, 178 and 219 are the "block characters".
Some modern text-based programs, such as "Dwarf Fortress", emulate text mode in a graphical mode, and use images of the classic PC font. You can find some of these bitmaps that you can use on the Dwarf Fortress Wiki see (user-made tilesets).
The Text Mode Demo Contest has more resources for doing graphics in text mode.
Here are the relevant settings:
pane-active-border-bg colour
pane-active-border-fg colour
Set the pane border colour for the currently active pane.
So, try adding something like this to your ~/.tmux.conf:
set-option -g pane-active-border-fg blue
That will set a blue border around the active pane. The pane-active-border-bg option can be used for a more visible solution, as well.
Best Answer
In tmux manual ($man tmux) you can find this section:
So, I think what you want is to use the '-l' flag, which is for switch to last pane. I don't know if there is a default key mapping defined to this task, but you can accomplish this by doing your own bindings. Something like this: