Drive buttons can become unreadable when customizing the theme with a valid color combination

[slauriere]

Steps to reproduce

Customize the following colors as follows in the wiki theme:

• btn-default-bg: #7a7a7a
• btn-default-color: #fff

Expected result

The Drive buttons have the theme colors (they are btn-default), or possibly custom colors but with a readable text.

Actual result

The Drive buttons have a gray color on a gray background.

This seems to be due to the fact that the DriveSheet defines the following custom CSS rules for btn-default, but without redefining the background-image rule, which is also customized by the theme (but do we really want to customize the btn-default styles at this level instead of using the global common style)?

.no-bootstrap .btn.btn-default {
  background-color: $theme.pageContentBackgroundColor;
  border-color: $theme.borderColor;
  color: $theme.textSecondaryColor;
}

Wdyt?