Fix rendering of compile-time and exec-time table

Background colour of pct_delta was not rendered on odd numbered lines,
because HTML bgcolor tag does not override CSS background-color style.
The patch changes cells to use standard compliant CSS style.


git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@208720 91177308-0d34-0410-b5e6-96231b3b80d8