In Manual 8.7:
https://www.tcl.tk/man/tcl8.7/TkCmd/ttk_style.html
DEFINITIONS: ... The “.” style is the theme root style on which derived styles are based.
or
https://tkdocs.com/tutorial/styles.html
# a example:
package require Tk
ttk::button .b0 -text Hello
ttk::button .b1 -text Hello
ttk::button .b2 -text Hello
ttk::button .b3 -text Hello
pack .b0 .b1 .b2 .b3
ttk::style configure Bold.TButton -font MPbold
ttk::style configure Emergency.Bold.TButton -foreground red -padding 10
ttk::style configure Emergency.TButton -foreground red -padding 10
.b1 configure -style "Bold.TButton"
.b2 configure -style "Emergency.TButton"
.b3 configure -style "Emergency.Bold.TButton"