fix: fixed styles for legacy xblock's editor

[bydawen]

There is a problem with a styles for the legacy xblock's editor in CMS:
openedx/frontend-app-authoring#1904

Currently, styles are used there that correspond to legacy theme, not to paragon design system.

Main goal of this PR is tweak current styles for greater consistency with paragon, and fix some style bugs (input and textarea styles, button colors, border colors, hover and focus styles etc.)

Some of fixed xblock's editors for example:

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[ihor-romaniuk]

[question] Why do we need to change variable names?

[ihor-romaniuk]

[question] I'm not sure that we need to change this import. What is the reason of this change?