zsh: let terminal set text colour

the prompt no longer sets the text colour which is better suited for the
terminal colour scheme, as when switching between light/dark mode, the
text does not blend into the background

stow/.zshrc

@@ -30,10 +30,9 @@ if [ -f ~/.cache/zsh_colours.sh ]; then
 else
   PRIMARY_COLOUR="{blue}"
   SECONDARY_COLOUR="{yellow}"
-  TEXT_COLOUR="{black}"
 fi
 
-PROMPT="%F$PRIMARY_COLOUR%~ %F$SECONDARY_COLOUR$OSICON %k%F$TEXT_COLOUR"
+PROMPT="%F$PRIMARY_COLOUR%~ %F$SECONDARY_COLOUR$OSICON %k%F"
 
 bindkey -e