Style changing does not work correctly in productive mode

[xmatthias]

Describe the bug
When building for production (yarn build) and running that version, each theme can only be selected once. Switching back to the previous theme is not possible.

To Reproduce
Steps to reproduce the behavior:

• select theme
• select different theme
• switch back to original theme (notice it stays with the theme from the step before).

In development mode, themes are imported directly (as text within the <style> tag) - and are then loaded inline into the style tag.
in production mode, it's a seperate css file - therefore the ID assignment fails, so disabling is not working.

I'm not certain how / if to fix this yet - as i think the different themes may cause display problems in conjunction with eCharts - which can only be "dark" or "light" - but not different shades of dark.

In the end, this might mean that we will fall back to 2 default styles (one light, one dark).
This might also simplify layout, as the different themes have quite different sizes (buttons / header line are higher in some styles).

@GrilledChickenThighs - as the author of the bootswatch theme part - have you got an opinion related to this?

[pAulseperformance]

Hey there, I will be gone camping for the next few weeks and will work on this issue when I return -- if it hasn't been solved by then.

Not sure what the issue might be as of yet, been a few months since I fired up freqtrade.

[xmatthias]

enjoy! 👍

[xmatthias]

Before working on this - please also have a look at #49 - i'm not certain we need all possibilities, maybe using the flatly / darkly options (to have a bright / dark) style will be enough.

Having the UI overall look good on all possible Versions will be very difficult ... so we might need to verify one or the other ...