Styling layout ux polish part 1

[christophidesp]

Theme colors (light and dark)

• Light theme: mainBackground and inputs are white, borders are one step darker, and panelBg and topBarBg are lighter.
• Menu items: selected and hover states are neutral grey instead of green.
• New tokens:
  • tabActiveBackground and tabItem keep tabs green.
  • panelBgGrad is a horizontal gradient. The background mixin outputs background: for gradient values, so it works as a prop.
• Warning color is darker: yellow80 (#FF9700) → yellow70 (#DF8400) in light, yellow60 (#BF7100) in dark.
• Dark theme: text is a little dimmer, dropdowns are darker, table row and column highlights are flat greys, and dropdownTable was removed.

Table

• Tables get an outer border and rounded corners. The new bordered prop (default true) turns the border off.
• Header bar: gradient background, bottom border, [2, 4] padding.
• Column header row: panelBg background and a fixed 40px height.
• Row hover uses tableRowBgHover.
• Column visibility button:
  • Uses the new columns icon with a "Columns" tooltip, and adds a columnVisibilityFlavour="hollow" prop.
  • Sets aria-expanded while its menu is open.
  • The menu title is now "Show columns" (was "Edit columns").

Button

• Neutral hollow buttons use inputBorderHover for the hover and active border, instead of primary.
• aria-expanded="true" applies the active styles, so a button looks pressed while its dropdown is open.

Dropdowns, tabs and select

• Dropdowns: a new dropdownStyles.js exports dropdownBorder and dropdownRound. Any Drop with background="dropdown" gets a border and rounding by default.
• Tabs:
  • Horizontal padding is [0, 4]; vertical (column) tabs keep [0, 0.5].
  • The -1px bottom margin was removed.
  • In the light theme, tab backgrounds are transparent, including warning and error tabs. The dark theme is unchanged.
• Select: option hover uses menuItemHover.

Icons

• Added add_more_charts, columns, dashboard_import and nav_reset.

Dependency: https://github.com/netdata/cloud-frontend/pull/5793