Fix markers in sign column are hard to read.

[kflu]

This fixes altercation/solarized/issues/252.

[kflu]

Validated this fix for all variations of visual settings (dark/light, contrasts, visibility etc.)

Before:

After:

[andyleejordan]

Instead of .s:bg_base03, how about .s:bg_back so it appears any changes made to the background color? It would then be the same as applied to the normal color group.

[ericbn]

I fixed this in my fork: https://github.com/ericbn/vim-solarized

Using bg_base02 (same used by LineNr) instead of bg_base03.

[178inaba]

@altercation Hello. Can you confirm this pull request?

There is a similar pull request. #172, #83, #77, #62