When picking a custom diff color in the user preferences, the **gutter border color doesn't change**. As explained in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/78281#note_888573538 by @wwwjon, this was a compromise in the implementation of https://gitlab.com/gitlab-org/gitlab/-/merge_requests/78281+: > As I wrote above, this is the compromise for the "Gutter background" and the "Gutter border". I checked the colors of the "Gutter border" of all themes: Only the "Light" theme uses two separate colors, which depend on the diff colors. All other themes use a single color that does not depend on the diff colors. So my suggestion is to not override the "Gutter" border color when a custom diff color is set.