Wrong layout for TheBlogTheme Search/NotFound pages

[tbolon]

Describe the bug

When using TheBlog theme, the /Search and /Not-Found theme pages are using a wrong header definition.

The html div header container class attribute is missing "position-relative" class, which results in grayed title due to CSS and overlay and some other layout problems.

To Reproduce

Steps to reproduce the behavior:

1. Create a new CMS app
2. Browse to /Search or any non-existing page
3. The color of the title of the page is gray instead of white

Expected behavior

The page title color should be white instead of gray, the test should be centered.

The culprits seems to be the liquid shared views which are not using the same HTML as the Content.liquid template.

Screenshots