Qudi light theme

[Geegee]

Alongside the dark theme add the option for a light theme.

Description

At the moment just the style sheet + images.
Once Gui files are in place will add the commits
to make the theme configurable through config file
and settings gui of the manager.

Motivation and Context

How Has This Been Tested?

Most things worked. Just check boxes were invisible.

On linux x11 window manager.

Screenshots (only if appropriate, delete if not):

Types of changes

• Bug fix
• [x ] New feature
• Breaking change (Causes existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• I have documented my changes in /docs/changelog.md.
• My change requires additional/updated documentation.
• I have updated the documentation accordingly.
• I have added/updated the config example for any module docstrings as necessary.
• I have checked that the change does not contain obvious errors
  (syntax, indentation, mutable default values, etc.).
• I have tested my changes using 'Load all modules' on the default dummy configuration.
• All changed Jupyter notebooks have been stripped of their output cells.