Use Jenkins theme CSS color variables for inverse text

[jdknight]

The original support for negative/inverse text uses a combination off currentColor and a bright-level-7 value. When using a default Jenkins theme, these color definitions work as expected. However, clients using the dark theme may not be able to read the text since both the text and background are a bright color.

Modern Jenkins themes provide CSS color variables. Instead of trying to rely on currentColor, use the default text color (var(--text-color)) and the default background color (var(--background)). This should allow negative/inverse text to render appropriately no matter what theme mode is used.

Testing done

With unit test changes that seemed to be right (please check), perform a mvn test to confirm expected results.

Started up a fresh Jenkins container instance and setup a pipeline job with the following:

node {
    stage('Build') {
        ansiColor('xterm') {
            echo 'This is a test.'
            echo '\033[7mThis is a test.\033[0m'
            echo 'This is a test.'
        }
    }
}

Before the changes, installed the ansicolor plugin from the online and observed the issue:

Then uninstalled the plugin, restarted and manually installed a newly built ansicolor.hpi plugin. Navigated back to the job results and observed that the inversed text could be read:

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[dblock]

Very nice! Add a line to https://github.com/jenkinsci/ansicolor-plugin/blob/master/CHANGELOG.md?

Let's increment the version to 1.1 as part of this PR as well, it's a big change.

[jdknight]

Add a line to https://github.com/jenkinsci/ansicolor-plugin/blob/master/CHANGELOG.md?

I do not mind adding some information there, but it seemed that as of v1.0.3, change log entries are only being populated inside GitHub releases.

[dblock]

Add a line to https://github.com/jenkinsci/ansicolor-plugin/blob/master/CHANGELOG.md?

I do not mind adding some information there, but it seemed that as of v1.0.3, change log entries are only being populated inside GitHub releases.

ah yes, thank you