Problem:
The base font in Confluence, as specified in main-action.css is font-family: Helvetica,Arial,sans-serif;
If a user has Helvetica installed on his system, Confluence will use this (ugly!) font.

Improvement: In our opinion Arial should always be the default font, so --> font-family: Arial,Helvetica,sans-serif;

This applies to two styles:

main-action.css, line 5:

body, p, td, table, tr, .bodytext, .stepfield
{
font-family: Helvetica,Arial,sans-serif;
font-size: 10pt;
line-height: 13pt;
color: #000000;
font-weight: normal;
}

main-action.css, line 1256:

input
{
font-family: Helvetica,Arial,sans-serif;
font-size: 10pt;
color: #000000;
}