The Guided Tour widget and step popover don't follow the Color Theme

The Guided Tour styles use hard-coded colors instead of the Color Theme variables, so the widget and the step popover keep a white background and grey text whatever Color Theme is used, including dark ones.

Hard-coded colors are found in:

• guidedtour-xwiki/src/driverjs.css and guidedtour-ui/src/components/widget/styles/driver.css, for the step popover: #fff, #2d2d2d, #727272, #d2d2d2, #ccc, #f7f7f7
• GuidedTourWidget.vue: background-color: white
• GuidedTourWidgetProgressBar.vue: #7698ffff for the progress bar, which doesn't follow the primary color of the theme
• GuidedTourWidgetUsefulLink.vue: #ddddddff
• styles/variables.css: -guidedtour-text-color: #b0b0b0 and -guidedtour-background-color-secondary: #f2f2f2, which are not based on the Color Theme either

These should use the Color Theme variables, the same way the rest of the skin does.

This is the same problem as XWIKI-21968 on the Tour application.