Steps to reproduce. Add the following markup to a Confluence page:

{panel:title=Panel|bgColor=lightblue} 
{table} 
||Col1||Col2|| 
|Foo|bar| 
{table} 

{panel} 
{table} 
||Col1||Col2|| 
|Foo|bar| 
{table}

Switch between the default theme and the documentation theme. When using the Documentation theme, the table gridlines disappear in the table inside the panel