Hi @Fatemah,
This plot is showing level 7 (i.e., species level) — just not all are assigned to species, and some are assigned to species that have no species-level annotation in the reference database. See this post and search elsewhere on this forum for more details.
With such a large number of species, there is no way to design a color scale that can accommodate all of them. Even if you did, e.g., use a color gradient, human eye cannot easily distinguish so many colors. You can choose different color palettes (with up to 20-some colors), which have been designed (by others not 
) for color-blind accommodation and general readability. These are the color palettes available in the plotting packages that uses to generate these plots.
SVG is not browser-specific. You can open and edit SVGs with all sorts of image editing software — try Adobe Illustrator or Inkscape, maybe gimp would work too.
Given your concerns about the colors, though, I recommend that you download the data as CSV, filter out very low abundance taxa and others that you don't want to plot, and then plot either using R or even in excel so that you can customize each individual color. (of course you can also do the filtering in , then use image editing software to manually recolor bars).
Check out the other tutorials — moving pictures is just a taste of some basic analyses. Check out the overview tutorial, which gives a good beginner-level overview of many different other methods and tutorials.
Or see here for a list of all available plugins and plugin actions.
Good luck!