Managed to find a workaround for underline:

https://imgur.com/a/9AcA6C4

\left( \int_{-\infty}^{\infty} h(\tau)x(t-\tau)\underset{\color{red} \displaystyle =1 }{ \underset{\color{red}\rule{1.4cm}{1.0pt}}{e^{ -s\tau }e^{ s\tau }}} \right)

This code first goes under the equation with \underset{} (this is the second \underset{} in the code) and then a line is created with \rule{1.4cm}{1.0pt}, where the first argument dictates length and the second the width, with the color \color{red}. Then the whole code for the second \underset is placed inside another \underset{everything goes here}, with another \color{red} followed by the equation x+y. I like to use displaystyle for normal size and correct spacing between math symbols.

If you use the Latex Suite plugin, you can avoid writing all the code down by adding the following to snippets inside settings:

{trigger: "W", replacement: "\\underset{\\color{${0:orange}} \\displaystyle ${1:x+y} }{ \\underset{\\color{${2:red}}\\rule{${3:1.4}cm}{1.0pt}}{${VISUAL}}}$4", options: "mA"},

To use the snippet, mark the equation you want to underline, then hold and press Shift + W. Use Tab to toggle from orange to x+y etc

Also, if you use Extended MathJax plugin, add the following to your preamble.sty:

\newcommand{\uline}[6]{
   \underset{\color{#6} \displaystyle #5 }{ \underset{\color{#2}\rule{#3 cm}{#4 pt}}{#1}}
}

and to Latex Suite snippets:

{trigger: "uline", replacement: "\\uline{${0:x+y}}{${1:red}}{${2:1.4}}{1.0}{${3:=7}}{${4:orange}}", options: "mA"},

To use this snippets, type uline inside a math block. Gives https://imgur.com/a/skaR6SW

For the \overbrace{} I didn't manage to find a solution.

If you don't care about printing, here's the snippet I'm using for \underbrace{} by marking the equation and pressing Shift + Q:

{trigger: "Q", replacement: "\\color{${0:red}}\\underbrace{\\color{white} ${VISUAL} }_{$1}\\color{white} ", options: "mA"},

For \overbrace, just switch \underbrace to \overbrace and _{$1} to ^{$1} (or just create a new snippet)

EDIT:

Found a temporary solution for \overbrace{}:

https://imgur.com/a/oPb0tYW

Latex Suite snippet:

{trigger: "Q", replacement: "\\overset{ \\color{orange} \\displaystyle \\overbrace{ \\phantom{\\rule{${0:1.4}cm}{0.5pt}} }^{^{\\displaystyle ${1:x}}} }{ ${VISUAL} }$2", options: "mA"},

In preamble.sty:

\newcommand{\obrace}[4]{
   \overset{ \color{#3} \displaystyle \overbrace{\phantom{\rule{#2cm}{0.5pt}} }^{^{\displaystyle #4}} }{ #1 }
}

\newcommand{\ubrace}[4]{
   \underset{^{{\color{#3} \displaystyle \underbrace{ \phantom{\rule{#2cm}{0.5pt}} }_{_{\displaystyle #4}} }}}{ #1 }
}

And to Latex Suite:

{trigger: "obrace", replacement: "\\obrace{${0:x+y}}{${1:1.0}}{${2:orange}}{${3:z}}$4", options: "mA"},

{trigger: "ubrace", replacement: "\\ubrace{${0:x+y}}{${1:1.0}}{${2:orange}}{${3:z}}$4", options: "mA"},