UI issus for downhills

[houjoe0829]

1. help and feedback's close icon, and the close icon in the pop-up: https://www.figma.com/file/DGWHxU8cZHGT1pksAbRZlZ/virgo-2-1.0-editor-area(AFFiNE)?node- id=10587%3A15920&t=A4USl294NpqBz5u3-4
2. Shadow adjustment for dark mode sidebar: box-shadow: 4px 0px 24px #161616; https://www.figma.com/file/DGWHxU8cZHGT1pksAbRZlZ/virgo-2-1.0-editor-area(AFFiNE)?node- id=10525%3A16251&t=A4USl294NpqBz5u3-4
3. dark mode popup mask adjustment: #222222,60%; popup shadow adjustment (all uniform): box-shadow: 0px 4px 24px #161616; https://www.figma.com/file/DGWHxU8cZHGT1pksAbRZlZ/virgo-2-1.0- editor-area(AFFiNE)?node-id=10516%3A15860&t=A4USl294NpqBz5u3-4
4. Sidebar font colour code all use body colour #424149 (including account information)
5. dark mode hovertips background: #0C0A15
6. All the changes to the placeholder illustrations, new dark night mode (already talked to Jin Fei): https://www.figma.com/file/DGWHxU8cZHGT1pksAbRZlZ/virgo-2-1.0-editor-area(AFFiNE)?node-id=10846%3A17511
   The "used threads" colour code has been adjusted in both Pure White Angel (E3E2E4) and Dark Night Elf (3C3A40)