Custom theme seconday colour

[booth-w]

⚬ PROBLEM:

The other themes (plain, sunset, night, etc.) have a secondary colour for the top bar but custom themes do not.

⚬ SOLUTION:

I think there should be an option to set the secondary colour for custom themes to allow for a slimier design to the other themes.

⚬ ALTERNATIVES:

An option to add a user's own custom CSS.

⚬ RELEVANCE / SCOPE:

I doubt many people would use this feature but it does allow for a lot more customisability.

⚬ "SIDE EFFECTS":

Settings imported from a previous version would not have the option `theme_secondary_color` but this could be fixed by just setting the secondary colour to what the user had as their primary. ⚬ _CONTEXT_:

// Thank you!

SHORT Table	(Summary)
Problem	Missing custom theme secondary colour
Solution	Adding a secondary colour option in the custom theme
Alternatives	User's custom CSS
Scope	Few
Side effects	Could break settings imports from old versions
Context

[ImprovedTube]

hi! @booth-w looking forward for our themes' code to be finished, so that several more colors scan be set manually too.