The MantineReactTable defaults table cell backgrounds to
var(--mantine-color-body) which is opaque. This makes the
downloads widget table block appear solid while the footer
and other widgets correctly show the board's background.
Added a styles.css targeting table elements to force
background-color: transparent, following the same pattern
used by the beszel-system-table widget. Also added
bg=transparent to mantinePaperProps for the Paper wrapper.
Thomas