Update tmux config for 3.0+

It's quite easy to get 3.0+ on most distros of Linux nowadays. Sorry in
advance if you're using an older version.

If a large demand for supporting 2.x comes in, there are options to
situationally set specific variables based on the current tmux version.

An example of that can be found here: https://stackoverflow.com/a/40902312

.tmux.conf

@@ -1,5 +1,6 @@
 # -----------------------------------------------------------------------------
-# This config is targeted for tmux 2.1+ and should be placed in $HOME.
+# This config is targeted for tmux 3.0+ which is painless to install on
+# Ubuntu 20.04 LTS (official apt repo), Debian Buster (backports) and Arch.
 #
 # Read the "Plugin Manager" section (bottom) before trying to use this config!
 # -----------------------------------------------------------------------------
@@ -43,20 +44,21 @@ setw -g pane-base-index 1
 set -g mouse on
 
 # Colors optimized for Atom One Dark.
-set -g status-fg colour110
-set -g status-bg default
+set -g status-style fg=colour110
 
-set -g pane-border-fg colour240
-set -g pane-active-border-fg colour243
+set -g pane-border-style fg=colour240
+set -g pane-active-border-style fg=colour243
 
-set-window-option -g window-status-current-fg colour39
+set-window-option -g window-status-current-style fg=colour39
 
 set -g status-left ''
+set -g status-left-length 0
+set -g status-right ''
 set -g status-right-length 0
 
 # Display a clock on the bottom right of the status bar.
-#set -g status-right-length 20
 #set -g status-right '%a %Y-%m-%d %H:%M'
+#set -g status-right-length 20
 
 # -----------------------------------------------------------------------------
 # Key bindings