How to change the design of tab in terminator?

Update: terminator also uses gtk in background so check your version of terminator using:

terminator -v

If it is version 0.97 then make changes in the gtk2 file because it uses gtk2, and Ubuntu uses gtk3 so you have to change in both files.

For the theme check use:

gsettings get org.gnome.desktop.interface gtk-theme 

then change the following in /usr/share/themes/<YOUR_THEME>/gtk-2.0/gtkrc:

style "notebook_bg" {
  bg[NORMAL] = shade (1.02, @bg_color)
  bg[ACTIVE] = shade (0.97, @bg_color)
  fg[ACTIVE] = mix (0.8, @fg_color, shade (0.97, @bg_color))
}

Replace @bg_color in bg[NORMAL] with a color of your choice, e.g., "#F98686"(quotes required).

To make changes in gtk3, open ~/.config/gtk-3.0/gtk.css (if it doesn't exist then create it):

TerminalWindow .notebook tab:active{
  background-color: @bg_color;
}

Save it and reopen the terminal.

For other change :

right click on terminator screen --> preferences 

Here you can change terminator appearance that you want to change like custom profile, layouts, plug-ins, colors, etc. Once you add a new configuration it will create configuration files under ~/.config/terminator/.

Edit : for color change :

preferences --> profiles--> colors 

for layout change :

preferences --> profiles--> Layouts--> Add--> give name --> close

It will create config file ~/.config/terminator/config open this file using

gedit ~/.config/terminator/config

Change size and other things that you want in this file. You can also change color from this file.

reference :link

Note : Kindly re-launch the application for the changes to take effect.

The accepted answer seems to be quite outdated and doesn't work with newer Ubuntu versions.

To change the look of tabs in terminator have a look at the following post:

https://stackoverflow.com/questions/50082359/terminator-1-91-change-tab-theme-color-using-gtk3-css

The following settings worked for me:

/* ~/.config/gtk-3.0/gtk.css */

.terminator-terminal-window notebook tab {
  background-color: darkgrey;
  padding: 0;
  border: 0;
}
.terminator-terminal-window notebook tab:checked {
  background-image: none;
  background-color: #459cc4;
}
.terminator-terminal-window notebook tab button {
  padding: 0;
}

The following blog shows some more customization settings:

http://blog.nabam.net/workstation/2017/09/15/terminator_tabs