You can add the following code to your notebook, which will apply to all cells in the current notebook regardless of what cell it's entered in:
%%HTML
<style type="text/css">
table.dataframe td, table.dataframe th {
border-style: solid;
}
</style>
If you want it to apply to all notebooks, you can add a custom config/css file. Answers on how to do that can be found here.
You might also want to explore jupyterthemes depending on how much other configuration you want to do.
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