With the latest update a new feature was introduced, the "Custom CSS" rules.
This allows to write your own stylesheet extensions, which you can use to further adapt Kimai to your own needs.
You find the full documentation at https://www.kimai.org/documentation/plugin-custom-css.html
Two basic examples to trigger your imagination:
Removing the red dotted lines between overlapping timesheet entries
table.dataTable tr.overlapping {
border-top: none;
}
Highlight active timesheet records
tr.recording {
background-color: #ffa059 !important;
}