[feature] Add option to change the theme and add custom theme

[grgprarup]

Description

Currently, the slide's theme defaults to the white theme of revealjs. Add an option to choose the theme of user's choice.

Available slide themes in reveal: beige, black-contrast, black, blood, dracula, league, moon, night, serif, simple, sky, solarized, white-contrast, white.

Tasks to do

• Adapt our slide's theme according to the dark and light theme of web shifted to [feature] Change slide theme according to oc web theme #17
• Add option to change slide theme

[koebel]

Here is a brief user story to clarify the different aspect of what is meant by template and theme,

As a user
I want to choose a theme that applies to the design of all slides of my presentation 
So that I can ensure that the whole presentation has a consistent particular look

As a power user
I want to create my own theme(s) 
So that I can ensure that the design of my presentations complies with my company's / organisation's corporate design

By design and look we mean consistent use of

• a certain color palette
• font(s)
• and optionally other design elements

throughout all slides of a presentation

The chosen theme should be specified in the .md file through a text variable, not in the UI of the presentation viewer

[koebel]

changing the OC theme from light to dark mode or vice versa shouldn't affect how the slides are rendered (background colour, font colour, etc. ), only UI elements of web.
see also #17 (comment)