Is there a way to change the default Background color to a different color so that all new reports would use that color for the field property? And is there a way to have the line underneath the Column Title expand to the length of the field rather than just the length of the column name?

The background is a function of the stylesheet used so if you change the background color in default.sty you'll probably get what you need.

As for the other question, the easy answer is no. You can put the column titles in headings/subheadings and then draw lines where you want them, but the default is to underline only the width. Just like if you use styling to underline an A20 field that only contains 5 characters. It only underlines what is displayed, not the whole length of 20 characters.

For the underline of the title, have you tried the stylesheet option BORDER.

TABLE FILE CAR
PRINT CAR
ON TABLE SET HTMLCSS ON
-*ON TABLE PCHOLD FORMAT PDF
-*ON TABLE PCHOLD FORMAT EXL2K
ON TABLE SET STYLE *
 GRID=OFF, $
 TYPE=TITLE, BORDER-BOTTOM=LIGHT,STYLE=-UNDERLINE, $
ENDSTYLE
END