Unstable site after playing with wp-admin files

Ok, so courosity has gotten the best of me and I have begun to tinker with a few of the files found in the wp-admin direcory. Mostly just admin-header.php, admin-footer.php, index.php, and index-extra.php. I haven’t changed anything of great importance, just a few design fetures and some of the wordings. So now after messing around with these files my site is very unstable, could this be because I have made a few minor edits to these files? I am having a hard time understanding why this would be the case. the only edits i made had to do with the wording in he admin menu, and how the footer and dashboard looks. Nothing inricale to the workings of WP. Is this the issue? Or could it be something else?