XWIKI-21253: Improve the Gradient of buttons

[manuelleduc]

Jira: https://jira.xwiki.org/browse/XWIKI-21253
Forum discussion: https://forum.xwiki.org/t/gradient-on-buttons-minimalist-skin-design-1/12618/
Follow up work on image borders: #2319

Draft PR

I will document below some before/after screenshot of some elements of XWiki Standard with and without gradients on buttons.

Comment button

Base

before

after

Hover

before

after

Active

before

after

Disabled

after

Edit page buttons

The state change are on Save & View.

Base

before

after

Hover

before

after

Active

before

after

Disabled

after

[Sereza7]

Looks good to me 👍

Do you think we could remove the .btn-gradient(@color) mixin? I couldn't find any other occurence of it used in the xwiki organisation on github and I don't think it would make much sense for an extension or a custom script to use it because we already have 5 button variations that should be used instead of implementing a whole new button type.

EDIT: fixed a mistype .btn-background(@color) --> .btn-gradient(@color)

[manuelleduc]

Do you think we could remove the .btn-gradient(@color) mixin? I couldn't find any other occurence of it used in the xwiki organisation on github and I don't think it would make much sense for an extension or a custom script to use it because we already have 5 button variations that should be used instead of implementing a whole new button type.

I've considered it and:

1. it's been there since so long I would say it's part of the API (but we could agree to break it)
2. this can still be useful for whoever creates a theme with gradient buttons on top of flamingo