The plugin doesn't use symbol but outdated svg that are not themable

https://github.com/jenkinsci/bitbucket-branch-source-plugin/tree/master/src/main/webapp/images

For example in dark mode it's not very visible compared to other symbol

Symbols must be added on src\main\resources\images\symbols

See also: https://weekly.ci.jenkins.io/design-library/symbols/

Symbol can also be consumed from core or other plugin like https://github.com/jenkinsci/oss-symbols-api-plugin/tree/main/src/main/resources/images/symbols