Update README for all themes

Contains the following changes:

- Screenshots for each theme
- Instructions on how to update the Action Bar background
- Note at bottom asking to refer main README on how to use the theme
files
- Light version of search icon
Update default theme with the blue tint

The light theme’s colours were changes from green to blue. Since the
default theme is supposed to be the same theme, making the same
green-to-blue changes to the default theme.
Change header text colour to white

Also revert the accidental change to childView background colour.
Triangle in chat bubble

Also, fix for a few more issues that were caused during the theme
change.
@nalinchhajer1's fix for "fragment not yet attached" issue

Fix "fragment not yet attached" issue in SearchFragment for lower
versions
Fix attachment image bug

When reporting an issue, the add attachment icon should appear when you
remove the preview of an attached file. i.e. hs_add_attachment_img
instead of hs_attachment_img
Rename showGear method in HSHelpStack to showHelp
Prevent FAQ heading from overlapping > icon.