Make table inherit body's color

[bblanchon]

When trying to fix #1308, I quickly realized that hardcoding the values $table-color doesn't work great for dark color mode.

The best solution I found was to reset the value of $table-color like so:

$table-color: initial !default;

The effect of this is that the table inherits the color of the body, which works well for all themes in both light and dark modes.

[thomaspark]

Thanks for the PR. This seems on the right path, but there is some inconsistency with font color when a class like .table-primary is applied.

For example, Pulse before:

Pulse after:

And for reference, Pulse v4:

[bblanchon]

I amended my previous commit.
Here is the new Pulse:

I updated all the other themes too.

[thomaspark]

Looks good, thank you!