Class diagram theming: taming the default colors

Default class diagrams are too loud for company docs. Here’s how to tune skinparam for a calmer look.

The default theme problem

The out-of-the-box class theme uses saturated fills and heavy borders. Fine on white, but too loud in slides or on dark documentation sites.

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@startuml
class Animal
class Dog
class Cat
Animal <|-- Dog
Animal <|-- Cat
@enduml

A calm skinparam set

Drop these right after @startuml to override the defaults:

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@startuml
skinparam class {
  BackgroundColor #FAFAFA
  BorderColor #2F4858
  ArrowColor #2F4858
  FontColor #1F2328
  HeaderBackgroundColor #E8EEF4
}
skinparam stereotypeCBackgroundColor #FAFAFA
skinparam stereotypeABackgroundColor #FAFAFA
skinparam classAttributeIconSize 0
hide circle

class Animal {
  +name: String
  +age: int
  +makeSound()
}
class Dog {
  +breed: String
  +bark()
}
class Cat {
  +indoor: boolean
  +meow()
}
Animal <|-- Dog
Animal <|-- Cat
@enduml

Key bits:

• BackgroundColor #FAFAFA — off-white, softer than pure white.
• BorderColor #2F4858 — dark blue-gray; all lines use it.
• HeaderBackgroundColor #E8EEF4 — header row tinted.
• classAttributeIconSize 0 — drops the tiny lock icon.
• hide circle — removes the extra circles on inheritance arrows (UML2 doesn’t draw them).

Dark-background variant

If your docs site is dark, swap the palette:

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@startuml
skinparam class {
  BackgroundColor #161B22
  BorderColor #58A6FF
  ArrowColor #58A6FF
  FontColor #E6EDF3
  HeaderBackgroundColor #21262D
}
skinparam stereotypeCBackgroundColor #161B22
skinparam stereotypeABackgroundColor #161B22
hide circle

class Animal {
  +name: String
  +age: int
}
class Dog
class Cat
Animal <|-- Dog
Animal <|-- Cat
@enduml

Share via a theme file

If most of your diagrams use the same palette, move the skinparams into themes/puml-theme.puml and !include it:

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!include themes/puml-theme.puml
@startuml
class Animal
class Dog
Animal <|-- Dog
@enduml

Vendor the theme file in CI — no network fetch.

Debug workflow

Draw a 2-class empty diagram first; flip skinparam values one at a time until it looks right. Then build the real diagram.