Enables theme support

[kirchsth]

related to #245 and #291,

C4-PlantUML is prepared that themes can be used (all internal variables are only set, if they are not defined in an e.g. theme). This enables custom themes like below

@startuml
!theme C4_FirstTest from https://raw.githubusercontent.com/kirchsth/C4-PlantUML/extended/themes
!include https://raw.githubusercontent.com/kirchsth/C4-PlantUML/extended/C4_Component.puml

Person_Ext(customer, "Customer", "A customer of Widgets Limited.")

Enterprise_Boundary(c0, "Widgets Limited") {
    Person(csa, "Customer Service Agent", "Deals with customer enquiries.")
    System(ecommerce, "E-commerce System", "Allows customers to buy widgts online via the widgets.com website.")
    System(fulfilment, "Fulfilment System", "Responsible for processing and shipping of customer orders.")
}

System_Ext(taxamo, "Taxamo", "Calculates local tax (for EU B2B customers) and acts as a front-end for Braintree Payments.")
System_Ext(braintree, "Braintree Payments", "Processes credit card payments on behalf of Widgets Limited.")
System_Ext(post, "Jersey Post", "Calculates worldwide shipping costs for packages.")

Rel_R(customer, csa, "Asks questions to", "Telephone")
Rel_R(customer, ecommerce, "Places orders for widgets using")
Rel(csa, ecommerce, "Looks up order information using")
Rel_R(ecommerce, fulfilment, "Sends order information to")
Rel_D(fulfilment, post, "Gets shipping charges from")
Rel_D(ecommerce, taxamo, "Delegates credit card processing to")
Rel_L(taxamo, braintree, "Uses for credit card processing")
Lay_D(customer, braintree)

SHOW_LEGEND()
@enduml

The sample theme C4_FirstTest is stored in the themes folder.

The list of all changeable variables is documented in the Themes.md.

The MR is basically full working (it can be checked via my extended branch), but atm the documentation is not finished and I have no useful theme therefore it is a draft.

@Potherca, @manonmichel, @ProtoSlayer: it should fix #245 and #291. Can you please check if everything is OK or if I have missed something.

AND if you have a nice theme (best case based on an existing theme) maybe you want to publish it that it can be included.

BR Helmut

[kirchsth]

Another open question is the integration in the stdlib, details see https://forum.plantuml.net/17601/how-where-should-integrate-themes-into-the-plantuml-stdlib

[Potherca]

I have some minor wording issues and a question or two...

[Potherca]

Should we do "something" regarding defaults? I was looking at the current default settings:

Name	Size
LEGEND_DETAILS_SMALL_SIZE	10
ARROW_FONT_SIZE	12
STEREOTYPE_FONT_SIZE	12
TECHN_FONT_SIZE	12
LEGEND_DETAILS_NORMAL_SIZE	14
EIGHT_SIDED_SIZE	18
ROUNDED_BOX_SIZE	25

Name	Color
COMPONENT_FONT_COLOR	#000000
LEGEND_TITLE_COLOR	#000000
NODE_FONT_COLOR	#000000
PERSON_BORDER_COLOR	#073B6F
PERSON_BG_COLOR	#08427B
SYSTEM_BG_COLOR	#1168BD
CONTAINER_BORDER_COLOR	#3C7FC0
SYSTEM_BORDER_COLOR	#3C7FC0
CONTAINER_BG_COLOR	#438DD5
BOUNDARY_COLOR	#444444
LEGEND_DARK_COLOR	#66622E
ARROW_COLOR	#666666
EXTERNAL_PERSON_BG_COLOR	#686868
COMPONENT_BORDER_COLOR	#78A8D8
COMPONENT_BG_COLOR	#85BBF0
EXTERNAL_PERSON_BORDER_COLOR	#8A8A8A
EXTERNAL_SYSTEM_BORDER_COLOR	#8A8A8A
EXTERNAL_SYSTEM_BG_COLOR	#999999
NODE_BORDER_COLOR	#A2A2A2
EXTERNAL_CONTAINER_BORDER_COLOR	#A6A6A6
EXTERNAL_CONTAINER_BG_COLOR	#B3B3B3
EXTERNAL_COMPONENT_BORDER_COLOR	#BFBFBF
EXTERNAL_COMPONENT_BG_COLOR	#CCCCCC
SKETCH_BG_COLOR	#EEEBDC
ELEMENT_FONT_COLOR	#FFFFFF
LEGEND_FONT_COLOR	#FFFFFF
NODE_BG_COLOR	#FFFFFF
SKETCH_WARNING_COLOR	red
BOUNDARY_BG_COLOR	transparent

And I was thinking, the "theme" can be greatly simplified by providing a few COLOR and SIZE settings and filling the rest with calculated values (lighter/darker and smaller/larger). At the heart of the matter only 2 values would be needed (or three if the black/white also needs to be another value).

Just thinking out loud though.

[kirchsth]

@Potherca: related to "dependent color/size":

• Size should be simple I can try to make a calculation (I try to find out the correct default size that I can make all other values relative)
• but related to the colors I'm not sure if there is a simple rule and if you check the (typical, reused) themes they use already concrete colors and e.g. $DARK_DARK could be lighter than $DARK

[kirchsth]

@Potherca:

related to "Size should be simple..."

I tried to introduce a default font size as starting point (that we can calculate all other sizes based on it) but skinparam defaultfontsize xxx didn't scale as expected therefore I didn't introduce it and for the moment I will leave it as it is.

If you review is OK I would merge it and would even create a new release 2.6 out of it (and move all other open into a backlog release?)

[Potherca]

Great Work!

[Potherca]

Regarding:

If you review is OK I would merge it and would even create a new release 2.6 out of it (and move all other open into a backlog release?)

I'm all for it!

Usually I'd say bump all "documentation" issues to a 2.6.1 (so we can follow up with writing some prose and docs but no code changes) and everything else to v2.7 but based on #293, do we want to make a 2023-Q2 milestone instead?

[mweagle]

Thanks for adding theme support! I've published a handful of themes based on ColorBrewer and Seaborn palettes to https://github.com/mweagle/C4-PlantUML-Themes if you're interested in including/linking. Thanks again.