Hello @catwingz,
To set a white color to the text of the table, place the below CSS on your additional CSS
Just go to Dashboard >> Appearance >> Customizer >> Additional CSS
.woocommerce-Tabs-panel table tr th,
.woocommerce-Tabs-panel table tr td {
color: #fff !important;
}
Change your desired color value to this given css.
and about your given CSS code you missed class selector (.) on woocommerce-Tabs-panel and also table has a id of size-guide so you either write table#size-guide or just #size-guide like
.woocommerce-Tabs-panel #size-guide thead th,
.woocommerce-Tabs-panel #size-guide tbody td {
color: #fff !important;
}
.woocommerce-Tabs-panel table#size-guide thead th,
.woocommerce-Tabs-panel table#size-guide tbody td {
color: #fff !important;
}
I hope it works for you!
Sincerely,
Minal