Add color variables for text on blurred background

[Pytal]

Adds variables required for nextcloud-libraries/nextcloud-vue#3307

Compared to --color-text-maxcontrast the new --color-text-maxcontrast-background-blur is darker in light mode and lighter in dark mode

Light mode: #767676 -> #646464
Dark mode: #8C8C8C -> #919191

[blizzz]

master is still 26, please trigger a backport if necessary

[szaimen]

@Pytal @skjnldsv do we need to add the variable to the default.css variables and to the other themes as well?

[skjnldsv]

@Pytal @skjnldsv do we need to add the variable to the default.css variables and to the other themes as well?

It should be dispatched to the DarkTheme.php as well as there is a darken usage (so should be lighten on dark themes)
Highcontrasts already extend the default light and dark theme, so we should be good here (unless the contrast is not enough, therefore require some tuning there too)

[Pytal]

/backport to stable25

[jancborchardt]

This is important for accessibility, could you review @skjnldsv @szaimen @juliushaertl @marcoambrosini? :)

[Pytal]

Also made sure that text remains high contrast in high contrast modes