Highlight currently selected concept in alphabetical list sidebar

[osma]

Reasons for creating this PR

The currently selected concept in the alphabetical index sidebar should be highlighted according to the design spec.
This one line CSS change implements this (issue #1563 task 1a).

Link to relevant issue(s), if any

• Part of Implement alphabetical view in sidebar using Vue #1563

Description of the changes in this PR

Modify CSS to make sure the currently selected concept is highlighted in the alphabetical index.

Known problems or uncertainties in this PR

None

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

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

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.94%. Comparing base (5c6ff5a) to head (edeecf9).
Report is 67 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1732      +/-   ##
============================================
- Coverage     73.13%   70.94%   -2.20%     
+ Complexity     1958     1650     -308     
============================================
  Files            33       33              
  Lines          4769     4330     -439     
============================================
- Hits           3488     3072     -416     
+ Misses         1281     1258      -23     

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[UnniKohonen]

LGTM!