A "theme" in delta is just a collection of settings grouped together in a named feature. One of the available settings is
syntax-theme: this dictates the colors and styles that are applied to foreground text by the syntax highlighter. Thus the concept of "theme" in delta encompasses not just the foreground syntax-highlighting color theme, but also background colors, decorations such as boxes and under/overlines, etc.
The delta git repo contains a collection of themes created by users. These focus on the visual appearance: colors etc. If you want features like
navigate, you would set that yourself, after selecting the color theme. To use the delta themes, clone the delta repo (or download the raw
themes.gitconfig file) and add the following entry in your gitconfig:
[include] path = /PATH/TO/delta/themes.gitconfig
Then, add your chosen color theme to your features list, e.g.
[delta] features = collared-trogon side-by-side = true ...
Note that this terminology differs from bat: bat does not apply background colors, and uses the term "theme" to refer to what delta calls
syntax-theme. Delta does not have a setting named "theme": a theme is a "feature", so one uses
features to select a theme.