Add style for disconnected BusbarSection

[So-Fras]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [x] Tests for the changes have been added (for bug fixes / features)

What kind of change does this PR introduce? Feature

What is the current behavior? There is no existing style for a disconnected BusbarSection.

What is the new behavior (if this is a feature change)? A css style is added for disconnected BusbarSection.

Does this PR introduce a breaking change or deprecate an API?

• [x] Yes
• [ ] No

If yes, please check if the following requirements are fulfilled

• [x] The Breaking Change or Deprecated label has been added
• [x] The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps) See the migration guide

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud