fix: Make the strikethrough in Markdown visible

[shenyfg]

The property text.strikethrough doesn't exist in the code(should be text.strike), causing strikethrough doesn't work in markdown.

Here's what it looks like.

Before fix

After fix

Furthermore, the default fg color of this color group is base00, which is too dark to see.
I changed it to base04, to make it more visable.