Hi,
thanks for your post, and sorry for the trouble.
Dequeuing the TablePress default CSS is not recommended, because it also contains important CSS for the JavaScript functions (like the positioning of the sorting arrow icons, and similar).
You should instead use more specific CSS selectors in your Custom CSS, to override unwanted TablePress default CSS. Note that TablePress even has a special textfield for this on its “Plugin Options” screen. For common examples, please see https://tablepress.org/faq/
Now, if you really must dequeue the TablePress CSS, that’s possible by using a plugin filter hook. Just add
add_filter( 'tablepress_use_default_css', '__return_false' );
to your theme’s “functions.php” file or a small custom plugin.
Regards,
Tobias