Assuming you have read project's Wiki, the most obvious thing to do is to define Themer block inside your i3's config.
Find a place where colors are defined, and wrap it with comments that themer will recognize, and you should get something like this:
# i3 config goes here...
# THEMER
set $background #1a1b26
set $background_alt #565f89
set $foreground #c0caf5
set $foreground_alt #f7768e
# THEMER_END
# i3 config continues here...
Don't think about these variables inside the block, it's fine to leave just 2 lines of comments: # THEMER
and # THEMER_END
Now we need to register i3 config inside Themer's config. By default it's located in ~/.config/themer/config.yml
If you haven't already, define some themes:
themes:
# Just a dummy themes, put any variables you want, not only colors!
dark:
background: #000000
foreground: #ffffff
light:
background: #ffffff
foreground: #000000
Then, define an i3 config file:
files:
i3:
# Themer can expand tildes!
path: "~/.config/i3/config"
comment: "#" # you can skip this one, because hash (#) is the default comment for Themer
format: "set $<key> <value>"
You can set reload command to automatically update your i3 environment after you set a theme:
# Put any command that can be executed via shell
reload: "i3 reload"
themes:
# ...
files:
# ...
You're good to go with this. You can check how themer recognizes your file by running this command:
$ themer files --check
> Listed configuration files:
>
> ok i3 (~/.config/i3/config)
To set a theme, run this command:
$ themer set <theme name>