JabRef 5.1--2020-08-25--2ce90ad
Linux 4.9.0-13-amd64 amd64
Java 15-ea

• Mandatory: I have tested the latest development version from http://builds.jabref.org/master/ and the problem persists

With the dark theme, if the "case sensitive" button is activated in the search bar, the search is case unsensitive.
And if the "case sensitive" button is deactivated in the search bar, the search is case sensitive.
Note: It works as expected in the light mode.

Steps to reproduce the behavior:

0. activate the light theme (and restart)
1. open https://github.com/JabRef/jabref/blob/master/src/test/resources/testbib/jabref-authors.bib
2. in the search bar, type "garrido"
3. if "case sensitive" is activated (i.e. "abc" is bold in the button), "No result found".
   if "case sensitive" is deactivated (i.e. "abc" is light in the button), "9 results found".
   (as expected)
4. now activate the dark theme (and restart)
5. in the search bar, type "garrido"
6. if "case sensitive" is activated (i.e. "abc" is bold in the button), "9 results found"
   
   if "case sensitive" is deactivated (i.e. "abc" is light in the button), "No result found"
   

So, in the dark theme:

1. the behavior is opposite to the one of the light theme
2. search is visually case sensitive when, in fact, it is not.