Saving the excel file in html encodes CSS as follows:
.xl32
{mso-style-parent:style0;
border-top:.5pt solid windowtext;
border-right:1.0pt solid windowtext;
border-bottom:1.0pt solid windowtext;
border-left:.5pt solid windowtext;}
.5 is displayed as a regular line, 1.0 as a thick line So I think setting te with .5pt should work ...