Update style of selected lines in commit and blob/file views

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As a follow-up to Update styling of selected line (!108262 - merged), where we updated the style of selected and commented lines in MR diffs. The commit diffs were slightly affected by those changes, and the blob/file view was not changed at all. This was an intentional deviation from the specs of the original issue, as commented in !108262 (comment 1270884521).

This issue is to maintain consistency between the different file views and their style of selected lines.