Prefix indictator doesn't show until reload

When tmux first loads, the indicator doesn't show up when pressed. However, if I do 
```
source-file ~/.tmux.conf
```
it suddenly works.

I should mention, I am using the tmux config file at https://github.com/gpakosz/.tmux, as well as TPM, so instead of doing 
``` set -g status-right '#{prefix_highlight} ```
I have to do
``` tmux_conf_theme_prefix="#{prefix_highlight}" ```