feat(lualine): darken lualine b section for better readability

[msc5]

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[nullchilly]

Before

This proposal

My take

IMO the previous lualine b section is indeed a bit harsh on the eyes. However, deleting the bg will make the section obsolete, e.g. Take a look at the lualine sections:

+-------------------------------------------------+
| A | B | C                             X | Y | Z |
+-------------------------------------------------+

After removing the bg for B it feels like:

+-------------------------------------------------+
| A | C                             X | Z |
+-------------------------------------------------+

With the above points I will strike in the middle and only darken the b section from surface1 to surface0