Refactor CSS names and merge colors: neutral background.

[isoos]

• Partial reimplementation of New naming scheme for theme roles + migrating some background colors. #8064, only changing one color role at a time, starting with the neutral (default) background.
• The neutral role could be thought as synonymous to default, but it is a bit more, e.g. the home screen's cards are also having the same background (over e.g. a grid or a grey background). Also, with the two themes, there is no default background, as the system settings may use the dark mode as default, hence the rename to neutral.
• The change merges dark mode's #303030 home card background with the neutral (default) background #212121. This is to follow the light theme, where the card has the same background as the default page. #212121 was later switched to darkGunmetal as Flutter website's new dark mode will use that.
• The new CSS role variable is used as a generic value for setting component's backgrounds, instead of having a separate variable for each component.

[isoos]

Note: applied darkGunmetal color from the Flutter website's upcoming dark mode (it was one of the named colors closest to what they are using).