Provide better contrasts for gruvbox-dark-medium.

[m-kru]

3c3836 is simply not enough. The window created by the LSP client is almost indistinguishable from the editor. Take a look:

Before:

After:

[jdinhify]

Thanks for this @m-kru

There's a few things:

• We should aim to not provide too much contrast in a "medium" contrast variant. If more contrast is needed, we should look at using (and improving) the hard contrast one. The points below are directly related to this.
• I do agree that the peekView background could be problematic for some, however the suggested colour is too bright. Could we please try increasing the opacity instead, such as #3c383680 or even #3c383690. Going lighter than that would not be ideal since it will significantly affect the contrast in the peekView.
• Please modify this PR to have the changes for the peekView only. The current border and highlighting background colours are intentionally used for medium contrast variant. Applying a lighter colour will need to be in a different discussion, and using hard contrast variant, again, would be the suggestion if more contrast is required.

It'd be nice to have the changes applied to all the variants. But I understand it will require much more work. So I'm happy for this PR to address the dark medium contrast variant only. We will create an issue to address other variants.